Allow Kestrel to listen on abstract UNIX socket

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

I would like to be able to configure Kestrel to listen on a UNIX socket in abstract namespace.

### Describe the solution you'd like

It would be great if `KestrelServerOptions.ListenUnixSocket` would accept a path to a socket that is not absolute but prepended with a `@` character, as is customary to denote an abstract UNIX socket.

### Additional context

Currently my workaround is to parse the configuration manually and explicitly create an `UnixDomainSocketEndPoint` with path from abstract namespace (starting with `\0` character). I pass the created endpoint directly to `KestrelServerOptions.Listen` method.

Start with KestrelServerOptions.ListenUnixSocket and compare its handling with the existing KestrelServerOptions.Listen entry point. Trace how the socket path reaches the endpoint, then verify that an @-prefixed path is accepted for an abstract UNIX socket while existing paths continue to work.

Written by the indexing model from the issue text.