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.

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

```diff @@ 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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