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GH-45358: [C++][Python] Add MemoryPool method to print statistics (#45359)

### Rationale for this change

Add a MemoryPool method to print allocator-specific statistics to stderr, to help diagnose perceived memory consumption issues.

Also add missing Python bindings for `MemoryPool::total_bytes_allocated` and `MemoryPool::num_allocations`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.

* GitHub Issue: #45358

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>