[Draft] Implement Conditional Updates for FlatPostgresCollection

[suddendust]

Description

This change implements Collection#UpdateResult update(Key key, Document document, Filter condition) for FlatPostgresCollection.

[codecov]

Codecov Report

❌ Patch coverage is 73.91304% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.64%. Comparing base (3504ce4) to head (b3e5f4b).

Files with missing lines	Patch %	Lines
...documentstore/postgres/FlatPostgresCollection.java	73.91%	13 Missing and 5 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #275      +/-   ##
============================================
- Coverage     80.71%   80.64%   -0.07%     
- Complexity        0     1398    +1398     
============================================
  Files           241      241              
  Lines          6928     6996      +68     
  Branches        642      652      +10     
============================================
+ Hits           5592     5642      +50     
- Misses          919      932      +13     
- Partials        417      422       +5     

Flag	Coverage Δ
integration	80.64% <73.91%> (-0.07%)	⬇️
unit	52.37% <0.00%> (-0.52%)	⬇️

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