Replace go-based WebTransport echo server with Rust implementation

[thomaseizinger]

Talking about this here: https://github.com/libp2p/rust-libp2p/tree/master/wasm-tests/webtransport-tests/echo-server

Depends on: #2993.

[mxinden]

👍

Any reason why not to add "help-wanted" label here @thomaseizinger?

[thomaseizinger]

👍

Any reason why not to add "help-wanted" label here @thomaseizinger?

It is not actionable now so I didn't want to draw unnecessary attention to it. I only recorded this so that I can link to it from the WASM support tracking issue. Perhaps it should have just been an in-line task for now.