GitHub Actions execution

Runs conformance suite daily / on manual trigger.

* For each implementation (plug / cowboy / uberauth)
* For latest implementation Hex release (stable / beta) / current `main`
* For latest `oidcc` Hex release (stable / beta) / current `main`

Creates `.zip` files as artifacts that can be used for certification.

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Run credo / formatter on commit / pr.

Start by reviewing the repository's existing GitHub Actions configuration and the conformance suite entry points. Map the requested implementation and release combinations, scheduled and manual runs, certification zip artifacts, and commit/PR Credo and formatter checks. Done means each requested matrix run and artifact is produced and the checks execute on commits and pull requests.

Written by the indexing model from the issue text.