Allow disabling cross-namespace references

[somtochiama]

Introduce the flag --no-cross-namespace-refs (defaults to false) for allowing cluster admins to disable cross-namespace references to sources.

When the controller is run with --no-cross-namespace-refs=true and ImageUpdateAutomation.spec.sourceRef.namespace refers to a GitRepository in a different namespace than the ImageUpdateAutomation object, the reconciliation will fail with the AccessDenied reason.

On access denied the controller sends logs and events e.g

{"level":"error","ts":"2022-01-27T15:00:20.108Z","logger":"controller.imageupdateautomation","msg":"access denied to cross-namespaced resource","reconciler group":"image.toolkit.fluxcd.io","reconciler kind":"ImageUpdateAutomation","name":"imageupdateautomation-sample","namespace":"flux-system","error":"can't access 'GitRepository/default/sample-repo', cross-namespace references have been blocked"}

Part of flux2#2337

[stefanprodan]

LGTM

Thanks @somtochiama 🏅