Create GitHub App manifest for quick creation

[eleftherias]

For users that want to run their own Minder instance, or run Minder locally for testing, they will need to create a GitHub App. We can create a GitHub App manifest to make the creation process faster and less error prone.

This is a nice to have for https://github.com/stacklok/epics/issues/154.

[evankanderson]

@jhrozek may have one of these sitting around

[evankanderson]

This happens once in the development setup, and no one's been excited to do it.