[python] TileDBConfig type alias, misc. type annotations

[ryan-williams]

• We use Dict[str, Union[str, float]] throughout the codebase to mean "TileDB config"; this factors+names that.
• Made some types more explicit+precise in test_experiment_query.py.
• Reference some types directly (instead of as strings)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.51%. Comparing base (a71c3b7) to head (8afee2b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3317      +/-   ##
==========================================
+ Coverage   85.32%   85.51%   +0.18%     
==========================================
  Files          52       52              
  Lines        5514     5517       +3     
==========================================
+ Hits         4705     4718      +13     
+ Misses        809      799      -10     

Flag	Coverage Δ
python	85.51% <100.00%> (+0.18%)	⬆️

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Components	Coverage Δ
python_api	85.51% <100.00%> (+0.18%)	⬆️
libtiledbsoma	∅ <ø> (∅)