feat: handle webstreams in reply.send

[Uzlopak]

Resolves #5279

Well, i did not write a test yet, but I wanted to get from you some feedback first regarding the approach.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[climba03003]

Should not use non-documented symbol.
We may do a similar check like typeof payload.getReader === 'function'.

[climba03003]

You should not change the payload here.
Readable.fromWeb must provided inside onSendEnd.

[mcollina]

...also, I think there are a few headers that should be preserved if it's a fetch Response.

[climba03003]

It would be another PR, but identify Response would be hard.
I don't know which function would be specific enough to identify it.

[Uzlopak]

Why on onSendEnd?

What if in an onSend hook i check for a stream like it is done in @fastify/throttle? Imho must be done as early as possible.

[climba03003]

It looks like a partial support of WebStream as we mutate the response in the middle.
I know that object will be serialized and BufferLike is already changed to Buffer.

But, WebStream should be special treated to allow better support or extendible like extends it to Response in the middle.
Ultimately, WebStream should be possible to do the same as NodeStream or even Response.
nodejs/node#42529

fastify.addHook('onSend', async (request, reply, payload) => {
  if (payload instanceof ReadableStream) {
    // we can use Response here
    return new Response(payload, {
      headers: { "Content-Type": "text/html" }
    })
  }
  return payload
})

[Uzlopak]

Closing in favor of #5286