Add support for upserts in FlatPostgresCollection

[suddendust]

Description

This PR adds support for upserts in FlatPostgresCollection.

Testing

Added integration tests to validate the correct behaviour.

Checklist:

• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.98%. Comparing base (3504ce4) to head (a8ed79d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #277      +/-   ##
==========================================
+ Coverage   80.71%   80.98%   +0.27%     
==========================================
  Files         241      241              
  Lines        6928     6954      +26     
  Branches      642      643       +1     
==========================================
+ Hits         5592     5632      +40     
+ Misses        919      905      -14     
  Partials      417      417              

Flag	Coverage Δ
integration	80.98% <100.00%> (+0.27%)	⬆️
unit	54.55% <93.33%> (+1.67%)	⬆️

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