feat: Add release candidate validation

[anniefu]

Runs buildpack integration tests before the release-please GitHub Action
creates a release candidate PR.

See functions-framework-conformance builidpack integration workflow PR for more information.

[grant]

Can you add a detailed PR description?

What is this really testing and what type of failures can we now uniquely detect with this action?

Is there an overall design for this validation work? There seems to be a lot of hardcoded values here.

[anniefu]

Can you add a detailed PR description?

What is this really testing and what type of failures can we now uniquely detect with this action?

Is there an overall design for this validation work? There seems to be a lot of hardcoded values here.

Added the link to the functions framework conformance PR that describes the integration test.

The only hardcoded value that is unfortunate is the builder tag, which can hopefully be removed if GCF tags their builders with latest. Otherwise, the values you see input are the same as required for conformance.yml or any conformance test run.

[grant]

LGTM. I'm not sure how this will orchestrate in practice, but we can try it out.