Introduce namespace management in ArtifactGenerator

[matheuscscp]

Closes: #388

This PR introduces two things that are tightly coupled: the ability to generate namespaces from .spec.artifacts[].namespace, and also making this field aware of .spec.pathPattern. Together, these two behaviors allow Flux users to generate namespaces from directories in source artifacts.

We are also aligning garbage collection behavior with kustomize-controller, which puts back objects into the inventory and keeps the finalizer when the AG is on deletion in case the API server does not confirm the deletion request for the object. Backoff retries from controller-runtime reattempt the finalization.

The API:

apiVersion: source.extensions.fluxcd.io/v1beta1
kind: ArtifactGenerator
metadata:
  name: my-ag
  namespace: my-ns
spec:
  commonMetadata: # Now gets applied to both EAs and Namespaces.
    ...
  namespaces:
    strategy: Managed # defaults to Unmanaged
    # prune: true     # true is the default
  sources:
    - alias: repo
      kind: OCIRepository
      name: monorepo
  pathPattern: ...
  artifacts:
    - name: ...
      namespace: ... # Resolves to "some-tenant".
status:
  inventory:
    - kind: Namespace
      name: some-tenant
      # Hash of the metadata applied by the AG.
      digest: sha256:589303e73217c92ae0d20d90d425cd143459e2ddc488aa38684d5e69e8c9b4b9

The namespaces generated by an ArtifactGenerator are the union of the namespaces rendered by all .spec.artifacts[]. The field .spec.commonMetadata is used for the metadata of the namespaces. If needed later, we can add .spec.namespaces.metadata to give users an override that is special for namespaces. Drift is detected against the inventory for garbage collection and against the metadata fields for correcting the remaining namespaces. Conflicts with other ArtifactGenerators and observability are similar to how we did it for ExternalArtifact.