Generalizing `use_github_action()` with `use_github_file()`

[malcolmbarrett]

I'm very interested in the pattern used by use_github_action() as an alternative to use_template() when it makes sense to host files somewhere else (e.g. we want to make it easier to grab a file from R but don't necessarily want to host it in the package itself because it's useful for other purposes).

I think it would be nice to refactor that internal code to a new exported function, e.g. use_github_file(), which could then be called by use_github_action(). The pattern for reading a file from GitHub is, thankfully, not that complex, but it's very useful.

Happy to prepare a PR if it seems like it's worth exporting that code

[hadley]

Yeah, a PR would be appreciated thanks.

[jennybc]

Very related to #1483