Only run failed jobs on internal test re-runs

[joshua-spacetime]

Description of Changes

Internal tests no longer dispatch a new private workflow if there's a linked private PR. Instead they report the results of the linked private workflow. If there is no linked private PR, it will dispatch a new private workflow. However if any of the jobs within that workflow fail, re-running the internal test will only run those failed jobs, assuming the refs haven't changed.

The purpose of this is to cut down on the number of redundant CI jobs/workflows and to make private test flakes much less costly for public changes.

API and ABI breaking changes

N/A

Expected complexity level and risk

3

Testing

• Linked private PR with private CI in progress: Internal tests wait on private CI to finish and report its result
• Linked private PR with private CI failure: Internal tests will re-run only the failed jobs
• Linked private PR with private CI success: Internal tests will not re-run CI, they will report the successful result
• Linked private PR but public SHA doesn't match: Error message (please update submodule)
• Public only change with no matching workflow: Dispatch new internal tests workflow
• Public only change with matching workflow: Report success or re-run failed jobs only

[jdetter]

Nothing blocking from me on this, we'll sync about it during standup with @bfops

[joshua-spacetime]

@bfops I've decided to rescope these changes to just the original proposed feature. As a result, I've removed the GithubApi wrapper entirely.

[bfops]

a bunch of nits and one small change. otherwise, LGTM