Add support for getting a GitHub App enterprise installation

[stevehipwell]

GitHub has announced support for apps to discover if they're installed in an enterprise and if so what the installation ID is. We should support this endpoint.

[danyalahmed1995]

Great Idea. I can take a look at this as a follow-up after #4225 settles, unless someone is already working on it. Probably not today but tomorrow 😆.

[gmlewis]

Thank you, @danyalahmed1995! It's yours.

[danyalahmed1995]

@gmlewis Aye aye Captain 🫡