Postgis join hints

[SpacemanPaul]

Reason for this pull request

For complex queries (as can occur when using search_returning), SQLAlchemy was non-deterministically failing to resolve a join strategy. This was caused some tests to fail intermittently. A re-run (or 2, or 3) of the tests was usually sufficient to bypass but it shouldn't have been happening and needed to be addressed.

Proposed changes

• Improved type hints in driver.postgis._fields
• Made NativeFields smart enough to generate an onclause parameter for the SQLA join function, and wire in to the query generation code.

Running a temporary minimal test that triggered the issue, this change took us from failing roughly 1 run in 4 to not failing at all for 25+ sequential test runs.

• Tests added / passed
• Fully documented, including docs/about/whats_new.rst for all changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.57%. Comparing base (5b0d0c7) to head (2c96d9f).
Report is 1 commits behind head on develop-1.9.

Additional details and impacted files

@@               Coverage Diff               @@
##           develop-1.9    #1564      +/-   ##
===============================================
- Coverage        85.62%   85.57%   -0.05%     
===============================================
  Files              140      140              
  Lines            15299    15366      +67     
===============================================
+ Hits             13100    13150      +50     
- Misses            2199     2216      +17     

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[Ariana-B]

Looks reasonable to me.