Relay server: support listening on both WebRTC and WebSocket

Description

Proposed solution:
Add configuration and implementation to allow the relay server to bind to both WebRTC and WebSocket addresses.
Use case:

Clients behind different NAT/firewall setups.
Improved interoperability with different types of libp2p implementations.
Related work:

Demonstration of JavaScript WebRTC peers connecting through the Rust relay server:
js-libp2p-examples Issue #226
js-libp2p-examples PR #227

Motivation

This work is part of a comprehensive test of WebRTC support across different libp2p implementations. The current focus is on enabling JavaScript peers to connect via a Rust relay server. In the future, the goal is to extend these tests to include scenarios where JavaScript and Rust private peers can interoperate using WebRTC through the relay. Supporting both WebRTC and WebSocket transports on the relay server will improve connectivity options and interoperability for clients using different protocols.

Current Implementation

Currently, the relay server does not listen on either WebRTC or WebSocket transports. This is needed for JavaScript peers—and, in the future, Rust peers using WebRTC—to connect to the relay server.

Are you planning to do it yourself in a pull request?

Yes

Start by locating the relay server's transport configuration and listener entry point, then review the linked JavaScript examples issue and pull request for interoperability expectations. Done means the relay can be configured to bind both WebRTC and WebSocket addresses and support clients using either transport.