Add support for GCS auth in both resolution and jar download phases

[acecilia]

Add support for GCS auth in both resolution and jar download phases:

• Resolution phase:

  • Gradle resolver:
    • The resolver now converts gcs:// to https://storage.googleapis.com/
    • Auth token is obtained using the google-auth-library-oauth2-http library. GcpTokenProvider wraps the calls to the library
  • Maven resolver:
    • GcsDownloadService SPI plugin uses google-cloud-storage library, and delegates non-GCS URIs to the default service. HttpDownloaderTransporterFactory now accepts gcs scheme
    • Auth token is obtained by the google-cloud-storage library

• Jar download phase:

  • download_pinned_deps.bzl converts gcs:// to https:// inline and uses Bazel's http_file
  • Auth token is obtained via Bazel --credential_helper (delegated to the client to configure). Example of credential helper here