remote: Add support for Google Cloud Storage.

Add support for Google Cloud Storage (GCS) as a HTTP caching backend.
This commit mainly adds the infrastructure necessary to authenticate
to GCS.

Using GCS as a caching backend works as follows:
  1) Create a new GCS bucket.
  2) Create a service account that can read and write to the GCS bucket
     and generate a JSON credentials file for it.
  3) Invoke Bazel as follows:
    bazel build
      --remote_rest_cache=https://storage.googleapis.com/<bucket>
      --auth_enabled
      --auth_scope=https://www.googleapis.com/auth/devstorage.read_write
      --auth_credentials=</path/to/creds.json>

I'll add simplification's and docs in a subsequent commit.

Change-Id: Ie827d7946a2193b97ea7d9aa72eb15f09de2164d
PiperOrigin-RevId: 179406380