feat(rbac): support direct group membership as superUsers

[Sancretor]

Hey, I just made a Pull Request!

This PR on RBAC plugin adds support for Group reference in superUsers configuration like below :

permission:
  enabled: true
  rbac:
    admin:
      superUsers:
        - name: user:default/alice
        - name: user:default/mike
        - name: group:default/admins # <--- this is new

A note has been added to state that this feature does not support transient memberships.

It implements feature discussed in #7783 🚀

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)
• All your commits have a Signed-off-by line in the message. (more info)

[backstage-goalie]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage-community/plugin-rbac-backend	workspaces/rbac/plugins/rbac-backend	minor	v7.7.1

[backstage-goalie]

Thanks for the contribution!
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