GS walker incorrectly uses MD5 instead of ETag for Google Storage objects

[guy-har]

Problem

The Google Storage walker uses MD5 field to identify objects, but some GS objects don't have MD5 values - specifically files created via compose operations.

Orignial comment

          LGTM beside a small comment, we need to fix the Walker implementation to list md5 and etag.

Originally posted by @nopcoder in #9041 (review)