[rust] Add streams WebTransport client and server examples

[cargopete]

Closes #479.

Adds streams-web-client and streams-web-server examples, matching the existing streams-nats-{client,server}, streams-quic-{client,server}, and streams-tcp-{client,server} pairs.

The WebTransport transport uses wtransport with a self-signed certificate (generated via rcgen at startup) and an Insecure cert verifier on the client side — the same pattern as the existing hello-web-{client,server} and wasi-keyvalue-web-server examples.

Both examples implement wrpc-examples:streams/handler.echo, echoing back a stream of numbers and a stream of bytes.

Running:

# terminal 1
cargo run -p streams-web-server

# terminal 2
cargo run -p streams-web-client "https://[::1]:4433"

Note: on macOS localhost may resolve to 127.0.0.1 rather than ::1, so pass the explicit IPv6 address when the server is bound to [::1] (the default).