feat(ext/fetch): add support for fetch on vsock sockets

[losfair]

This commit adds support for using vsock transport in fetch API on Linux and macOS.

Similar to #29154, a vsock transport can be specified in the proxy field when calling Deno.createHttpClient.

const client = Deno.createHttpClient({
  proxy: {
    transport: "vsock",
    cid: 2,
    port: 80,
  },
});

await fetch("http://localhost/ping", { client });

[lucacasonato]

This is missing a test that actually demonstrates a fetch happening over vsock - can you add that?