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 94.20290% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.11%. Comparing base (ef0d310) to head (3e43e9b).

Files with missing lines	Patch %	Lines
...documentstore/postgres/FlatPostgresCollection.java	94.20%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #275      +/-   ##
============================================
+ Coverage     80.98%   81.11%   +0.12%     
- Complexity        0     1446    +1446     
============================================
  Files           241      241              
  Lines          6954     7022      +68     
  Branches        643      653      +10     
============================================
+ Hits           5632     5696      +64     
  Misses          905      905              
- Partials        417      421       +4     

Flag	Coverage Δ
integration	81.11% <94.20%> (+0.12%)	⬆️
unit	54.79% <66.66%> (+0.24%)	⬆️

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