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 chart.spec.sourceRef refers to a GitRepository, Bucket or HelmRepository in a different namespace than the HelmRelease object, the reconciliation will fail with the AccessDenied reason.

On access denied errors the controller logs and sends an event e.g.:

can't access 'GitRepository/my-other-namespace/my-source', cross-namespace references have been blocked

And the HelmRelease status Ready condition it set to:

status:
  conditions:
  - lastTransitionTime: "2022-01-26T07:26:48Z"
    message: "can't access 'GitRepository/my-other-namespace/my-source', cross-namespace references have been blocked"
    reason: AccessDenied
    status: "False"
    type: Ready

Part of: fluxcd/flux2#2337

[stefanprodan]

LGTM

Thanks @somtochiama