feat(unstable): WebTransport (#27431)

Initial implementation of WebTransport client and server!

This is very unstable because the interface should eventually shift to
use hyper (h3 is on the [2025
roadmap](https://hyper.rs/contrib/roadmap/)) instead of manually messing
with the the protocol, which will enable integration with
Deno.serveHttp/etc and allow WebTransport over h2. This will also let us
expose multiplexing.

WebTransport stats will be a followup due to their complexity.

Fixes: https://github.com/denoland/deno/issues/9017

ext/net/lib.deno_net.d.ts

@@ -851,5 +851,17 @@ declare namespace Deno {
     options: ConnectQuicOptions<ZRTT>,
   ): ZRTT extends true ? (QuicConn | Promise<QuicConn>) : Promise<QuicConn>;
 
+  /**
+   * **UNSTABLE**: New API, yet to be vetted.
+   *
+   * Upgrade a QUIC connection into a WebTransport instance.
+   *
+   * @category Network
+   * @experimental
+   */
+  export function upgradeWebTransport(
+    conn: QuicConn,
+  ): Promise<WebTransport & { url: string }>;
+
   export {}; // only export exports
 }