Add russh::server::run_on_socket to facilitate dropping privileges immediately after socket binding

[samuela]

Securing SSH servers is not straightforward. Most state-of-the-art systems utilize privilege dropping after they bind to a socket in order to minimize their attack surface, including OpenSSH. Currently russh does not support this pattern since socket binding and the main server event loop are tied to a single function, russh::server::run.

This PR introduces a function russh::server::run_on_socket which is identical to russh::server::run except that it accepts a tokio::net::TcpListener in place of a socket address, allowing end-users to bind to a socket and perform other intermediate actions before handing over control to the main server event loop. russh::server::run's behavior is left untouched, so the change is fully backwards compatible.

[Eugeny]

Thank you!

@all-contributors add @samuela for code

[allcontributors]

@Eugeny

I've put up a pull request to add @samuela! 🎉