MCO-2031: OS Image Stream to install-config schema

This change introduces the OS Image Stream concept to the
install-config to facilitate day-zero stream selection.
The change adds an optional global field that can be used to set the OS
Image Stream to pick for all the pools and a per pool field to override
it or to just set the stream in a specific pool.

Author: pablintino

data/data/install.openshift.io_installconfigs.yaml

@@ -150,6 +150,15 @@ spec:
                   For the compute machine pools, the only valid name is "worker".
                   For the arbiter machine pools, the only valid name is "arbiter".
                 type: string
+              osImageStream:
+                description: |-
+                  OSImageStream specifies the OS Image Stream to use for machines in this pool.
+                  When set, this value overrides the global osImageStream defined in the InstallConfig.
+                  When unset, machines in this pool will inherit the global osImageStream value if configured,
+                  or fall back to the cluster's default OS image stream if neither value is set.
+                  This allows different machine pools to use different OS image streams while maintaining
+                  a common default for the cluster.
+                type: string
               platform:
                 description: Platform is configuration for machine pool specific to
                   the platform.
@@ -1710,6 +1719,15 @@ spec:
                     For the compute machine pools, the only valid name is "worker".
                     For the arbiter machine pools, the only valid name is "arbiter".
                   type: string
+                osImageStream:
+                  description: |-
+                    OSImageStream specifies the OS Image Stream to use for machines in this pool.
+                    When set, this value overrides the global osImageStream defined in the InstallConfig.
+                    When unset, machines in this pool will inherit the global osImageStream value if configured,
+                    or fall back to the cluster's default OS image stream if neither value is set.
+                    This allows different machine pools to use different OS image streams while maintaining
+                    a common default for the cluster.
+                  type: string
                 platform:
                   description: Platform is configuration for machine pool specific
                     to the platform.
@@ -3210,6 +3228,15 @@ spec:
                   For the compute machine pools, the only valid name is "worker".
                   For the arbiter machine pools, the only valid name is "arbiter".
                 type: string
+              osImageStream:
+                description: |-
+                  OSImageStream specifies the OS Image Stream to use for machines in this pool.
+                  When set, this value overrides the global osImageStream defined in the InstallConfig.
+                  When unset, machines in this pool will inherit the global osImageStream value if configured,
+                  or fall back to the cluster's default OS image stream if neither value is set.
+                  This allows different machine pools to use different OS image streams while maintaining
+                  a common default for the cluster.
+                type: string
               platform:
                 description: Platform is configuration for machine pool specific to
                   the platform.
@@ -4885,6 +4912,14 @@ spec:
                 - Internal
                 type: string
             type: object
+          osImageStream:
+            description: |-
+              OSImageStream is the global OS Image Stream to be used for all machines in the cluster.
+              This value serves as the default for all machine pools (control plane, compute, arbiter).
+              Individual machine pools can override this value by specifying their own osImageStream.
+              When both this field and the machine pool's osImageStream are unset, the cluster will
+              use its own default OS image stream.
+            type: string
           platform:
             description: |-
               Platform is the configuration for the specific platform upon which to

pkg/types/defaults/validation/featuregates.go

@@ -35,5 +35,32 @@ func GatedFeatures(c *types.InstallConfig) []featuregates.GatedInstallConfigFeat
 			}(),
 			Field: field.NewPath("compute", "diskSetup"),
 		},
+		{
+			FeatureGateName: features.FeatureGateOSStreams,
+			Condition:       len(c.OSImageStream) != 0,
+			Field:           field.NewPath("osImageStream"),
+		},
+		{
+			FeatureGateName: features.FeatureGateOSStreams,
+			Condition:       c.ControlPlane != nil && len(c.ControlPlane.OSImageStream) != 0,
+			Field:           field.NewPath("controlPlane", "osImageStream"),
+		},
+		{
+			FeatureGateName: features.FeatureGateOSStreams,
+			Condition:       c.Arbiter != nil && len(c.Arbiter.OSImageStream) != 0,
+			Field:           field.NewPath("arbiter", "osImageStream"),
+		},
+		{
+			FeatureGateName: features.FeatureGateOSStreams,
+			Condition: func() bool {
+				for _, compute := range c.Compute {
+					if len(compute.OSImageStream) != 0 {
+						return true
+					}
+				}
+				return false
+			}(),
+			Field: field.NewPath("compute", "osImageStream"),
+		},
 	}
 }

pkg/types/installconfig.go

@@ -228,6 +228,15 @@ type InstallConfig struct {
 	// E.g. "featureGates": ["FeatureGate1=true", "FeatureGate2=false"].
 	// +optional
 	FeatureGates []string `json:"featureGates,omitempty"`
+
+	// OSImageStream is the global OS Image Stream to be used for all machines in the cluster.
+	// This value serves as the default for all machine pools (control plane, compute, arbiter).
+	// Individual machine pools can override this value by specifying their own osImageStream.
+	// When both this field and the machine pool's osImageStream are unset, the cluster will
+	// use its own default OS image stream.
+	//
+	// +optional
+	OSImageStream string `json:"osImageStream,omitempty"`
 }
 
 // ClusterDomain returns the DNS domain that all records for a cluster must belong to.

pkg/types/machinepools.go

@@ -128,6 +128,16 @@ type MachinePool struct {
 	// The available types are etcd, swap and user-defined.
 	// +optional
 	DiskSetup []Disk `json:"diskSetup,omitempty"`
+
+	// OSImageStream specifies the OS Image Stream to use for machines in this pool.
+	// When set, this value overrides the global osImageStream defined in the InstallConfig.
+	// When unset, machines in this pool will inherit the global osImageStream value if configured,
+	// or fall back to the cluster's default OS image stream if neither value is set.
+	// This allows different machine pools to use different OS image streams while maintaining
+	// a common default for the cluster.
+	//
+	// +optional
+	OSImageStream string `json:"osImageStream,omitempty"`
 }
 
 // MachinePoolPlatform is the platform-specific configuration for a machine