Gold Pre-submit flow for contributors without permissions

[Piinks]

Description

There is currently a workaround in place to un-block first time/un-privileged contributors that cannot execute pre-submit tryjobs, but it does not allow these contributors to land new or updated golden file changes.
This is a more complete solution that removes that work around and provides a workflow for these contributors.

For permission-prohibited contributors, the presubmit comparator will instead use the goldctl imgtest check feature to lookup image hashes to confirm results. If a check fails, it will retrieve and use the raw expectations to confirm if it is a new test, allowing it to pass through, or if it is an expected change via Gold's ignore feature.

This very similar to the pre-tryjob implementation, without requesting images and manually comparing, and without the need to access Gold's details API.

Better error messages have been included in this change to provide more informative and actionable messaging in all of these use cases.

Related Issues

Fixes #46687
Fixes #47898

Tests

As an authorized contributor in this case, I reversed the logic on an earlier commit (and since reverted) to ensure that the presubmit checks completed correctly for the unauthorized use case.

• Restores all ignore tests from before tryjobs so that first-time contributors can make use of the feature
• The presubmit comparator makes the correct choice for authorized and unauthorized test cases
• The unauthorized comparator uses ignores to allow for expected changes and new tests

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read Handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.

[Piinks]

Trivial framework change to trigger the appropriate test shards.

[Piinks]

This is ready for review. @dnfield, this is the full solution for users without the proper permissions on Cirrus, and replaces the workaround I put in place before the holiday.

[dnfield]

Is this how we check elsewhere?

I think it would be better if we check the actual env variable that has the service account key and see if it starts with ENCRYPTED[ or something like that. That's what we really care about, right?

[Piinks]

Will do! :)

[dnfield]

LGTM