Add GitHub Action to test master rustfmt formatting vs a feature branch

This new action is intended to help us maintainers determine when feature
branches cause breaking formatting changes by running rustfmt (master)
and the feature branch on various rust repositories.

Over time I expect the list of checked projects to increase.

With this action in place we can more easily test that a new feature or
bug fix doesn't introduce breaking changes. Although this action needs to
be manually triggered right now, we might consider adding it to our CI
runs in the future.

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