Accept configured executor instances

[sapunyangkut]

Closes #801.

Summary

• accept an already-instantiated concurrent.futures.Executor in open_virtual_mfdataset
• leave caller-supplied executors open so callers can configure and reuse them
• retain executor-class support temporarily with a deprecation warning
• update the scaling guide, API documentation, release notes, and regression coverage

Validation

• python -m pytest virtualizarr/tests/test_parallel.py virtualizarr/tests/test_xarray.py::test_executor_instance_remains_usable -q (3 passed, 3 skipped)
• ruff check and ruff format --check on all changed Python files
• mypy virtualizarr/parallel.py virtualizarr/xarray.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.45%. Comparing base (8dc2cb7) to head (10edca6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1091      +/-   ##
==========================================
- Coverage   89.67%   89.45%   -0.22%     
==========================================
  Files          41       41              
  Lines        2682     2684       +2     
==========================================
- Hits         2405     2401       -4     
- Misses        277      283       +6     

Files with missing lines	Coverage Δ
virtualizarr/parallel.py	89.65% <ø> (ø)
virtualizarr/xarray.py	86.66% <100.00%> (+0.22%)	⬆️

... and 3 files with indirect coverage changes

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