Add `peer_addr` function to `UdpSocket`

[LinusU]

Long background: https://users.rust-lang.org/t/query-wether-a-udpsocket-has-been-connect-ed/26076

I think it would be nice if we could add the peer_addr (from TcpStream & UnixStream) to UdpSocket, this would work exactly the same as TcpStream (getpeername) and return the address that was previously passed to connect.

I'd be happy to try and take a stab at this! 🚀