Allow async callbacks in reply

[robertcepa]

This would solve...

I am using a library that proxies undici in an unintended way, changing the type of opts.body to AsyncGenerator. I would like to read the generator and reply with the result, but it's not possible because reply callback isn't awaited. This was discussed more in-depth here #1756.

This capability would also allow awaiting other async functions in the callback

The implementation should look like...

In my specific use-case, I'd be able to read AsyncGenerator and return the body string:

.reply(200, async (opts) => {
  const buffers = [];
  for await (const data of opts.body) {
    buffers.push(data);
  }
  const body = Buffer.concat(buffers).toString('utf8');
    
  return body;
});

[mcollina]

Thanks for reporting! Would you like to send a Pull Request to address this issue? Remember to add unit tests.