fix: provide websocket instead of stream to avoid potential backpressure issues (#289)

[mat-sz]

Now createWebSocketStream is not called therefore there are no memory leaks/backpressure to worry about.

Checklist

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

[mat-sz]

@mcollina As discussed here https://github.com/fastify/fastify-websocket/issues/289#issuecomment-1973621926

I've removed stream creation completely. Another option here is to provide an object of shape { socket: WebSocket } to provide compatibility with most common use cases.

Also, could be nice to provide a function to convert that socket into a stream; so the object could even be { socket: WebSocket; getStream: (opts?: DuplexOptions) => Promise<Duplex> }.

I can update this PR to include either of the aforementioned solutions.

[mcollina]

I don't think having a specific function is needed, let's just document how to create it using ws.

[mat-sz]

@mcollina I've added an example to README.md

[mcollina]

CI is failing

[mat-sz]

@mcollina Resolved, for some reason the unit test was relying on the count of fastify log items to be constant, updated it to fail if level >= 50 instead.