Provide bespoke thread-per-task-bounded-elastic scheduler

[bclozel]

The spring-graphql project is using a bespoke Scheduler when sending WebSocket messages to a WS session; this helps to guarantee that messages are sent in a serial fashion in blocking scenarios.

Community recently asked to use a Scheduler backed by virtual threads, when running on Java 21+. This solution works, but there might be room for improvement. With a newSingle scheduler, a virtual thread is kept alive for the entire duration of the websocket session, which is not a typical usage for VTs.

Reactor could provide a way to create a bespoke thread-per-task-bounded-elastic scheduler, with a custom ThreadFactory. This would allow to process messages in an efficient fashion when the task queue is full, and not hold onto a virtual thread when the queue is empty. We realize that reactor cannot mandate virtual threads on Java 21+ runtimes and that a system property must be configured to opt in. However, a custom ThreadFactory should be enough for spring-graphql to implement this.

We don't have performance benchmarks to prove a possible improvement, but this could be a nice addition to the Schedulers API.