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

❌ Patch coverage is 56.66667% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.58%. Comparing base (65909a2) to head (648e3c3).

Files with missing lines	Patch %	Lines
...documentstore/postgres/FlatPostgresCollection.java	56.66%	13 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #277      +/-   ##
============================================
- Coverage     80.68%   80.58%   -0.11%     
  Complexity     1396     1396              
============================================
  Files           240      240              
  Lines          6882     6908      +26     
  Branches        638      639       +1     
============================================
+ Hits           5553     5567      +14     
- Misses          914      926      +12     
  Partials        415      415              

Flag	Coverage Δ
integration	80.58% <56.66%> (-0.11%)	⬇️
unit	52.98% <0.00%> (-0.21%)	⬇️

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