Add well-known handler

[MarcoPolo]

Played around with this a bit and added a module to handle the .well-known/libp2p/protocols endpoint. The underlying idea is this enables other peers who discover us to understand what protocols we support and how to reach them. In a standard server/client deployment there is a lot of coordination on what protocol is where. With the well-known resource we can avoid this coordination and allow peers to learn about each other in a similar way to the identify protocol.

Example Caddyfile Usage:

:4002 {
	# Site-specific options
	bind multiaddr:/ip4/0.0.0.0/udp/4002/quic-v1/webtransport
	bind 127.0.0.1
	bind ::1

    handle / {
        respond "Hello World"
    }

	route /.well-known/libp2p/* {
		well_known-libp2p {
                        # Tell peers where our protocols are mounted
			/hello/0.0.1 => /
		}
	}
	respond "I am 4002"
}

$  curl 127.0.0.1:4002/.well-known/libp2p/protocols
{"/hello/0.0.1":{"path":"/"}}

Note that the user has to explicitly advertise protocols. I think this is okay, but could be better. I'm just not very familiar with caddy. Maybe it's possible to mark other routes/handlers with a decorator.