4.x: Inviting the Scheduler implementations from my RxJavaExtensions companion library. #8082

Proposal.

https://github.com/akarnokd/RxJavaExtensions?tab=readme-ov-file#custom-schedulers

• Shared - take an existing scheduler, one of its worker, and pretend that is by itself a full scheduler.
• Parallel - have an explicit sized pool of workers, smaller or bigger than Schedulers.computation().
• Blocking - blocks the current thread and runs tasks on the current thread. Could be useful within Virtual Threads now.

For Shared, you we could do ➕ Scheduler.share().
For the other two: ➕ Schedulers.newParallel(...) and ➕ Schedulers.newBlocking(...).

➡️ https://github.com/akarnokd/RxJavaExtensions/tree/3.x/src/main/java/hu/akarnokd/rxjava3/schedulers
➡️ https://github.com/akarnokd/RxJavaExtensions/tree/3.x/src/test/java/hu/akarnokd/rxjava3/schedulers