Support virtual threads in `webmvc.GraphQlWebSocketHandler`

[yassenb]

We are already reaping the benefits of virtual threads for regular GraphQL requests. It would be nice to have it working for GraphQL subscriptions as well. It should be as easy as replacing the SessionState scheduler with one that spawns virtual threads instead of platform ones for each websocket connection. Or am I missing something?

[vladimirfx]

Please consider back port this to 1.x release line.

Motivation
In one of our apps we use QraphQL based chat sessions. This leads to 300+ "GraphQL-WsSession-" threads in mid load and 700+ in busy hours. It is dominant native memory consumer for our app...

I tried to patch GraphQlWebSocketHandler locally with one liner:

this.scheduler = Schedulers.newSingle(Thread.ofVirtual().name(name).factory());

It works perfectly but that not a way for our project.

Alternatively provide some simple customization mechanics for this scheduler.