Pull in a GCR service key from input

[dmikusa]

Summary

This adds support to read a GCR service key from the action input if that is required to fetch images for the purpose of getting metadata to include in draft-release notes.

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).