Support comparing two test results

- Add result_utils that parse test results in XML or ZIP format.
- Add "--ref" option to "sheet" command that downloads test results from
  GCS and uploads comparison to Google Sheets.

Bug: 79905934
Test: sheet --src=test_result.xml --ref=gs:// --dest=
Change-Id: Id462d32136cef503e375bd4f937397c95b029e95