Implemented tools for interacting with GCS buckets.

Added functionality for listing all modules in a GCS bucket directory and downloading a module from a GCS bucket directory.

Change-Id: Iedacb01b328c51bceb21df50e86fbf88d99730fc
Reviewed-on: https://cos-review.googlesource.com/c/cos/cos-extensions/+/75253
Reviewed-by: He Gao <hegao@google.com>
Tested-by: He Gao <hegao@google.com>
Reviewed-by: Miri Amarilio <mirilio@google.com>