verify gcloud download with checksum

[jthegedus]

Is your feature request related to a problem? Please describe.

Currently the tar.gz of gcloud downloaded from GCS is not verified once downloaded. It should be checked against a hash to ensure it was downloaded correctly.

Describe the solution you'd like

GCP Storage APIs for GCS Object Get support fetching a crc32c and md5Hash field in the metadata. We should leverage those to ensure the file is correct once downloaded.

• GCS Object Get API Explorer: https://cloud.google.com/storage/docs/json_api/v1/objects/get
• GCE Object metadata docs: https://cloud.google.com/storage/docs/json_api/v1/objects#resource
• GCP checksum best practices: https://cloud.google.com/storage/docs/hashes-etags

Describe alternatives you've considered

NA

Additional context

Google recommends using crc32c over md5. Importantly, crc32c is NOT the same as crc32.

• crc32c installed via pip in the gcloud Google Cloud Builder: https://github.com/GoogleCloudPlatform/cloud-builders/blob/cb2df7028a2127d4ac0b8df34fc9d9fb8a5147ad/gcloud/Dockerfile.slim#L20