Vert.X: improve WebSocket performance

[kciesielski]

WebSockets in Vert.X have high latency overhead compared to other backends:

One fix that can be applied is similar to #3393
This requires:

• allowing a fast_path
• handling standard_path with more efficient stream merging. Use fs2.Channel to perform the merge (more efficient than Stream#mergeHaltL / Stream#parJoin), just as in the mentioned PR.
  This should be applied to the sttp.tapir.server.vertx.cats.streams.fs2.webSocketPipe function.

Verification:

1. vertxServer3/Test/testOnly *VertxServerTest*
2. Performance tests for Vert.X + WebSocketSimulation (See https://github.com/softwaremill/tapir/blob/master/perf-tests/README.md#testing-websockets)