[DPE-7379] custom username

[dragomirp]

Port of canonical/postgresql-k8s-operator#1060

Postgresql side of custom username implementation based on DA-176.

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.

[codecov]

Codecov Report

❌ Patch coverage is 36.50794% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.61%. Comparing base (cab9d43) to head (d11de2e).
⚠️ Report is 63 commits behind head on 16/edge.

Files with missing lines	Patch %	Lines
src/relations/postgresql_provider.py	32.75%	33 Missing and 6 partials ⚠️
src/charm.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           16/edge    #1100      +/-   ##
===========================================
- Coverage    65.09%   64.61%   -0.49%     
===========================================
  Files           17       17              
  Lines         4277     4329      +52     
  Branches       654      667      +13     
===========================================
+ Hits          2784     2797      +13     
- Misses        1318     1351      +33     
- Partials       175      181       +6     

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[dragomirp]

As is the oversee_users method will not delete custom usernames, since it cannot guess them and the relation should already be terminated. That means that re-relating with the same username will error out (the user already exists). We will need to port the logic for user deletion from k8s.

[marceloneppel]

LGTM!