Make axum generic over it's Listener type

[StratusFearMe21]

Motivation

I'd like making axum applications powered by Unix Sockets to be easier than having to make your own event loop from scratch. This PR allows you to put a UnixListener in axum::serve, or any other type that you implement Accept on.

Solution

This PR adds the Accept trait, which abstracts over the accept() function of the Listener type. This PR also adds the LocalAddr trait, which get's the associated Listener's Socket type's SocketAddr by calling the local_addr() function on the type. From there, changes were made all around the codebase to make sure that any type implementing Accept could be put into axum::serve

[StratusFearMe21]

The idea is that the new Accept trait that this PR introduces is as flexible as possible, so as of right now, it uses hyper::rt::Read + hyper::rt::Write as trait bounds. Im totally open to suggestions though about how we can replace these trait bounds with something non-hyper

[jplatte]

Did you see the existing issue at #2474 / PR at #2479?

[StratusFearMe21]

Huh, somehow I didn't see those

[jplatte]

That's unfortunate. I recommend next time you open an issue to discuss any non-trivial changes before making a PR. As for this change, is it okay if I close this as duplicate? Maybe your work on this could still be useful if it leads to any useful feedback on the other PR, if you want to check for any noteworthy differences and leave a review there that would be appreciated.

[StratusFearMe21]

Yeah, that's fine

Duplicate of #2474 / #2479