Support scoping Flagger to a specific list of namespaces

What problem are you trying to solve?

Currently, the Flagger operator can be configured to watch all namespaces (default) or a single namespace (via the -namespace flag). I'd like to enable the operator to be scoped to a list of namespaces.

### Proposed solution

Update the -namespace flag to accept a comma-separated list of namespaces (e.g., -namespace=ns-a,ns-b).