Migrate our shared github actions to a separate repository.

This will make PRs from forked repositories significantly less painful, since they'll agree on which version of each action to use.  OTOH, we'll have a separate repo that needs to be maintained, and changes to it will need to be coordinated and versioned carefully.  This will likely need to be done less often though now that our infrastructure is stable.

PiperOrigin-RevId: 512117705