feat(grpc): add server-side Listener and Transport traits

[sauravzg]

Server-owned connection lifecycle and graceful shutdown:

• Server accepts connections from a Listener and manages their lifecycle. Each accepted Transport is bound to a handler via Transport::serve(), which returns a GracefulConnection future that the Server polls and can shut down cooperatively.

• Graceful shutdown follows a pattern combining the tonic Server API with hyper's graceful shutdown guide (https://hyper.rs/guides/0.14/server/graceful-shutdown/).

• ListenerAddress provides a type-safe, downcastable address for listeners, analogous to Go's net.Addr.

Runtime API changes:

• Runtime trait gains tcp_listener() and unix_listener() methods (doc(hidden)) that return EndpointListener (pub(crate)).
• EndpointListener trait (pub(crate)) wraps accept() and local_addr() for server-side listening sockets.
• Tokio implementations: TokioTcpListener, TokioUnixListener.
• New address module (rt::address) with ListenerAddress trait and impls for SocketAddr and UnixListenerAddress.

Breaking changes:

• Server::serve() now takes &impl Listener (sealed) instead of the previous ServerListener. The old ServerListener returned a pre-assembled ServerCall with constructed streams; the new Listener yields raw Transport objects and stream construction moves into Transport::serve().

• serve_with_shutdown() is pub(crate) pending a design decision on whether the shutdown signal should be per-listener or per-server.

New public API surface:

• Dyn versions of handler, recvstream (pub) - Same as client so should be okay
• ListenerAddress trait (pub) — network(), as_any() - Needs to be public because it'll be exposed eventually
• Listener trait (pub, sealed) — accept(), local_addr()
• InMemoryServingConnection (pub)
• ListenerAddress impl for std::net::SocketAddr
• Runtime::tcp_listener(), Runtime::unix_listener() (doc(hidden))
• Transport trait (pub, doc(hidden), private token) — serve()
• GracefulConnection trait (pub, doc(hidden), private token) — graceful_shutdown()
• UnixListenerAddress (pub(crate))
• InMemoryListenerAddress (crate-private)
• EndpointListener trait (pub(crate))

Open Questions:

• local_addrs may need to be updated to be errorable(contrary to our previous discussion), tonic's API is errorable and we currently do an expect.
• shutdown per listener vs shutdown on server
• The use of associated types creates annoying issues of sealing and private token since dependent types need to be made public.