Make a github action for controlling checkout behavior

[mkruskal-google]

Right now, we need to manually checkout protobuf at a specific commit in every job in order to use our custom composite actions. We can't use an in-repo action to abstract away some of the specifics here, since until a checkout we don't have guaranteed access to repository files. Alternatively, we can create a separate repository for a protobuf-ci-checkout action. This would checkout the proper commit and initialize the repository for CI (e.g. setup .bazelrc files, fix newlines in windows, etc).