Start server on arbitrary free port or uds address

[hqdncw]

What is the problem your feature solves, or the need it fulfills?

I'd like the capability to launch a Pingora server on a random available port or UDS address and get back the port or address chosen.

Describe the solution you'd like

We can take inspiration from Tonic's implementation, which allows us to provide the listener after binding.

let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();

println!(listener.local_addr().unwrap().port());  // prints some random port assigned by the OS

tonic::transport::Server::builder()
    .add_service(svc)
    .serve_with_incoming(tokio_stream::wrappers::TcpListenerStream::new(listener))
    .await
    .unwrap();

Describe alternatives you've considered

...

Additional context

I need to launch multiple Pingora servers simultaneously for integration testing, each on a unique, dynamically assigned port or UDS address, to prevent port/address collisions.