fix(server): use the recommended connection handler from @fastify/websocket to avoid triggering stream backpressure (#5530)

[mat-sz]

Closes #5530

🎯 Changes

I've moved connection event handler creation into a separate function to allow for it being used directly inside of the @fastify/websocket connection callback.

Also, I deliberately left prefix handling in applyWSSHandler, so we can leave routing to fastify itself.

For future consideration - might be worth using something in line of the following to stay compatible with future versions of @fastify/websocket (if my changes get approved) alongside with backwards compatibility:

    fastify.get(prefix ?? '/', { websocket: true }, (connection, req) => {
      onConnection('socket' in connection ? connection.socket : connection, req.raw);
    });

There's a chance they might end up just exposing the raw WebSocket object on their end from the handler, but this isn't guaranteed to happen.

Update: I've submitted a PR to @fastify/websocket as well, might be worth checking out with regards to compatibility: fastify/fastify-websocket#290

✅ Checklist

• I have followed the steps listed in the Contributing guide.
• If necessary, I have added documentation related to the changes made.
• I have added or updated the tests related to the changes made.

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[KATT]

awesome, thanks

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