Add support for custom upstream docker registries in google_artifact_registry_repository

[rail]

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Description

The docker_repository.public_repository section of google_artifact_registry_repository supports DOCKER_HUB only, see https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/artifact_registry_repository#nested_docker_repository and https://github.com/hashicorp/terraform-provider-google/blob/1ba50ab27d4185c49a77ede83e665d1152fa6860/google/services/artifactregistry/resource_artifact_registry_repository.go#L361

Google supports custom registries. For example, you can specify "https://quay.io" and get it worked.

It would be great to allow arbitrary upstream registries.