Implement bulkUpdate for flat collections

[suddendust]

Description

This PR implements Collection#bulkUpsert for FlatPostgresCollection.

Testing

Added integration tests

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

Documentation

Note that based on the MissingColumnStrategy, we either skip the corresponding docs (and), or just skip those columns, or throw an exception.

[codecov]

Codecov Report

❌ Patch coverage is 87.87879% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.51%. Comparing base (faeac22) to head (b61e8ef).

Files with missing lines	Patch %	Lines
...documentstore/postgres/FlatPostgresCollection.java	87.87%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #269      +/-   ##
============================================
+ Coverage     80.44%   80.51%   +0.07%     
  Complexity     1396     1396              
============================================
  Files           239      239              
  Lines          6598     6663      +65     
  Branches        605      614       +9     
============================================
+ Hits           5308     5365      +57     
- Misses          888      894       +6     
- Partials        402      404       +2     

Flag	Coverage Δ
integration	80.51% <87.87%> (+0.07%)	⬆️
unit	54.93% <0.00%> (-0.55%)	⬇️

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