PMM-14433: recognize external postgres environment variables

[idoqo]

PMM-14433

Link to the Feature Build: SUBMODULES-4099

If this PR adds, removes or alters one or more API endpoints, please review and add or update the relevant API documentation as well:

• API Docs updated

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.70%. Comparing base (bed7077) to head (9f91155).
⚠️ Report is 2 commits behind head on v3.

Files with missing lines	Patch %	Lines
managed/utils/envvars/parser.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##               v3    #4719   +/-   ##
=======================================
  Coverage   45.69%   45.70%           
=======================================
  Files         363      363           
  Lines       37654    37656    +2     
=======================================
+ Hits        17207    17209    +2     
+ Misses      18790    18787    -3     
- Partials     1657     1660    +3     

Flag	Coverage Δ
managed	46.17% <0.00%> (-0.01%)	⬇️

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