Setting custom headers on initial WebSocket HTTP Upgrade response

[bahamut657]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

I'm building an HTTP reverse proxy using fastify.
In order to support other 3rd parts WebSocket connections, proxied by my software, I need a way to rewrite headers obtained in UPGRADE response from the replicated WS connection.
Is there a way to write some custom header in the initial upgrade request?

Example

In my case I have to replicate this headers:

Connection: upgrade
Content-Type: application/octet-stream
Date: Tue, 24 May 2022 16:19:40 GMT
Sec-WebSocket-Accept: MGhCOJFY8ayJUf3cJ+BcSgTwOOg=
Sec-WebSocket-Extensions: permessage-deflate
Sec-WebSocket-Protocol: tty
Server: Python/3.9 aiohttp/3.8.1
Upgrade: websocket

But from the reverse proxy I obtain this response:

Connection: Upgrade
Sec-WebSocket-Accept: 0hWXq3owSgRUjfyWd9dJpW6+lm0=
Sec-WebSocket-Protocol: tty
Upgrade: websocket

I have some missing fields and one to rewrite

[airhorns]

I don't think we support this right now and I took a quick look and it's not immediately obvious to me how we might add support. Its a bit tricky to write HTTP headers at the right moment before the connection becomes a bidirectional socket, but, it is definitely technically possible. Want to try a PR?

[bahamut657]

Sure, I can try a PR.
Could you suggest me the right starting point (files/mechanisms) to look at?