Translate MAP ctor and element access to JSONB in Postgres dialect

[ferenc-csaky]

Key changes

• MapConstructorSqlTranslation covers the ctor translation
• MapElementSqlTranslation covers the element access with a heuristic which will only work for textual keys for now

[codecov]

Codecov Report

❌ Patch coverage is 0% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 13.40%. Comparing base (5e4486c) to head (9596bf2).
⚠️ Report is 6 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ostgres/builtinflink/MapElementSqlTranslation.java	0.00%	15 Missing ⚠️
...res/builtinflink/MapConstructorSqlTranslation.java	0.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1802      +/-   ##
============================================
- Coverage     13.42%   13.40%   -0.02%     
  Complexity      734      734              
============================================
  Files           543      545       +2     
  Lines         15714    15735      +21     
  Branches       1829     1833       +4     
============================================
  Hits           2109     2109              
- Misses        13413    13434      +21     
  Partials        192      192              

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