[FEATURE REQUEST] Add isolate pools (thread pools)

[lohnn]

Is your feature request related to a problem? Please describe.
Currently integral_isolates only supports creating isolates manually, one at the time. If a developer was to increase the workload, by wanting to run more smaller jobs in parallell, or run different types of jobs from the same isolate, the back pressure strategy will not understand which jobs are connected to which intention. The work will also be quite slow, as they all share the same isolate.

Describe the solution you'd like
I'd like to investigate if we could implement an isolate pool to be able to spread the workload out.

Additional context
We will need to investigate and explore a few things:

• How do we decide how many isolates should be available in the pool?
  • Should the user/developer decide how many should be available?
    • Is there a global preference to set this?
    • Should the pool be local to just one instance of StatefulIsolate?
  • Should we automatically create threads when needed?
    • What is the upper limit of threads?
      • Maybe we peek at the CPU and create threads for best performance depending on physical threads?
• Should responses await each other to always return in the same order as the jobs that are added or should we let responses be out of order?
  • Maybe we could use tags as a way to set order?
• How to handle back pressure? Could we somehow piggyback on the queue, to decide when to run the BackPressureStrategy?

[lohnn]

• How do we decide how many isolates should be available in the pool?
  • Should the user/developer decide how many should be available?
    • Is there a global preference to set this?
    • Should the pool be local to just one instance of StatefulIsolate?
  • Should we automatically create threads when needed?
    • What is the upper limit of threads?
      • Maybe we peek at the CPU and create threads for best performance depending on physical threads?

[lohnn]

• Should responses await each other to always return in the same order as the jobs that are added or should we let responses be out of order?
  • Maybe we could use tags as a way to set order?

[lohnn]

• How to handle back pressure? Could we somehow piggyback on the queue, to decide when to run the BackPressureStrategy?