What is the problem I am trying to address?

Continuing the GCP storage backend lock removal, this PR aims to prevent regressions in future changes by precomputing the MD5 checksum and passing the checksum when uploading the file.

The computed checksum can be used for any storage provider; however, it is currently only implemented for GCP backend.

How is the fix applied?

• Further protect against corrupt cache by precomputing the MD5 sum for the ZIP file.
• Modify the Saver interface to accept the MD5 sum, and clarify the contract between the Saver implementation and the Saver user regarding the MD5 sum.
• Implement checksum in upload for GCP backend only.
  • Other backends have checksum passed to the implementation, but we did not implement passing the checksum to the upload to prevent breaking existing users since we have no way to test those implementations.

What GitHub issue(s) does this PR fix or close?

We are facing cache corruption in production.