feat: Implement shared memory pool for case where spark.memory.offHeap.enabled=false

[andygrove]

Which issue does this PR close?

Closes #996

Rationale for this change

Simplify memory configuration.

What changes are included in this PR?

Allocate one shared pool per executor, rather than one pool per native plan, when spark.memory.offHeap.enabled=false.

How are these changes tested?

• Existing tests
• Ran TPC-H benchmarks

[Kontinuation]

I'm a bit worried about this approach because we are implementing greedy mode inside CometTaskMemoryManager, which is known to starve consumers frequently. I prefer using fair spill pool for "native memory management" mode. This makes spillable operators work properly without being starved but with the cost of memory pool under-utilization.

[andygrove]

I'm a bit worried about this approach because we are implementing greedy mode inside CometTaskMemoryManager, which is known to starve consumers frequently. I prefer using fair spill pool for "native memory management" mode. This makes spillable operators work properly without being starved but with the cost of memory pool under-utilization.

Thanks for the feedback. I will work on a separate PR for the fair spill approach. I am moving this PR to draft for now.

[andygrove]

Closing in favor of #1021