Implement RFC 8414 for OAuth discovery

[Floppy]

I'd highly encourage you to also implement RFC 8414: https://datatracker.ietf.org/doc/html/rfc8414
You can see this in Mastodon here: https://github.com/mastodon/mastodon/blob/main/app/controllers/well_known/oauth_metadata_controller.rb
There is work to lift that up into doorkeeper, but I haven't had time to get back to it (funding): doorkeeper-gem/doorkeeper#1737

Originally posted by @ThisIsMissEm in #3951 (comment)