GCP sources spec update

BREAKING CHANGE: GCP sources parameter spec.serviceAccountKey (deprecated in #1316) is completely removed in favor of spec.auth.serviceAccountKey

Google Cloud Platform targets authentication spec update

Resolves #1463

BREAKING CHANGE: spec.credentialsJson is replaced with spec.auth.serviceAccountKey

This PR introduces IAM workload identity support for GCP targets on GKE. As a part of this update, the targets' specification was changed to be able to pass the required parameters and match the GCP sources' spec. Targets' spec.credentialsJson parameter was removed in favor of spec.auth.serviceAccountKey. GSA and KSA values can be passed through the spec' spec.auth.gcpServiceAccount and spec.auth.kubernetesServiceAccount params. spec.auth.kubernetesServiceAccount is optional and the controller will create either a new KSA (with a generated name if the attribute is not passed) or re-use existing if it is available.

Examples

Old target spec:

spec:
  credentialsJson:
    secretKeyRef:
      name: googlecloudpubsub
      key: creds

New equal version:

spec:
  auth:
    serviceAccountKey:
      valueFromSecret:
        name: googlecloudpubsub
        key: creds

IAM identity:

spec:
  auth:
    gcpServiceAccount: [email protected]

-controller will create KSA with generated name and annotate it with IAM SA value

spec:
  auth:
    gcpServiceAccount: [email protected]
    kubernetesServiceAccount: triggermesh-targets

-if KSA triggermesh-targets does not exist, the controller will create it and annotate it with the GSA value. Otherwise, it will assign existing KSA to the component's pod.

spec:
  auth:
    kubernetesServiceAccount: triggermesh-targets

-the controller will assign this KSA to the component's pod without the reconciliation.