Setting custom headers on initial WebSocket HTTP Upgrade response

### Prerequisites

- [X] I have written a descriptive issue title
- [X] 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