DRAFT: feat(engineio): add WebTransport transport support

[bneigher]

Summary

This PR adds WebTransport as an optional transport for Engine.IO connections, providing lower latency communication via HTTP/3 + QUIC.

Changes

• Add wtransport dependency with optional webtransport feature
• Create WebTransportTransport implementing AsyncTransport trait
• Add WebTransport variants to AsyncTransportType enum
• Add build_webtransport() and build_webtransport_with_upgrade() to ClientBuilder
• Add WebTransport-specific error types

Why WebTransport?

WebTransport offers several advantages over WebSocket:

• Lower latency via QUIC's 0-RTT connection establishment
• Support for unreliable datagrams (useful for real-time audio/video)
• Multiplexed streams without head-of-line blocking

Usage

// Enable the feature in Cargo.toml
// rust_engineio = { version = "0.6", features = ["webtransport"] }

let client = ClientBuilder::new(url)
    .build_webtransport()
    .await?;

// Or upgrade from polling if server advertises WebTransport support
let client = ClientBuilder::new(url)
    .build_webtransport_with_upgrade()
    .await?;

// Datagram support for unreliable low-latency data
client.transport().send_datagram(&audio_frame).await?;

Test plan

• Builds successfully with cargo build -p rust_engineio --features webtransport
• Builds without feature flag (no breaking changes)
• Integration tests with WebTransport server (tests marked #[ignore] - require server)