Add GCS support to skylark cp CLI command

[sarahwooders]

Tested with:

skylark cp gs://sarah-skylark-us-west1-a/test test
skylark cp test gs://sarah-skylark-us-east-1b/

Note that skylark cp test gs://sarah-skylark-us-east-1b fails (need / at end). Also both commands (below) copy the content of test (not the folder) into the bucket:

skylark cp test gs://sarah-skylark-us-east-1b/
skylark cp test/ gs://sarah-skylark-us-east-1b/

[parasj]

LGTM