feat: improve string_agg() for postgres

[Chris53897]

See beberlei#405

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.48%. Comparing base (9fd9c6f) to head (89c3701).
Report is 23 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #33   +/-   ##
=========================================
  Coverage     89.48%   89.48%           
  Complexity      548      548           
=========================================
  Files           190      190           
  Lines          2264     2264           
=========================================
  Hits           2026     2026           
  Misses          238      238           

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