[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 80.56%. Comparing base (57533ac) to head (ab31f4d).

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     80.63%   80.56%   -0.08%     
  Complexity     1396     1396              
============================================
  Files           240      240              
  Lines          6815     6883      +68     
  Branches        636      646      +10     
============================================
+ Hits           5495     5545      +50     
- Misses          907      920      +13     
- Partials        413      418       +5     

Flag	Coverage Δ
integration	80.56% <73.91%> (-0.08%)	⬇️
unit	53.17% <0.00%> (-0.54%)	⬇️

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