Support helm gcs repository and kustomize

Hi,

I would like to add following features:

Support for helm repository in Google Cloud Storage using helm gcs plugin (https://github.com/hayorov/helm-gcs). To authenticate with GCP, user can pass service account key via env variables (GCP_SERVICE_ACCOUNT_KEY).
Run Kustomize build before validate helm release.
Support multiple directories by providing comma separated list of directories or files.

Thanks.

stefanprodan / hrval-action