[DPE-8473] Handle invalid database name (databases)

[marceloneppel]

Issue

With the new predefined roles implemented for PG 16, if a client app requests a database called databases while the PG charm tries to set up the roles for it, a failure will happen because one of the roles will conflict with the already existing charmed_databases_owner role (responsible for owning all the databases).

Solution

This PR is a follow-up from canonical/postgresql-single-kernel-library#18.

The data directory permissions were fixed, as it was done on canonical/postgresql-operator#1156. Some more fixes across this charm codebase might need to be done to make everything consistent.

Another thing that was done in this PR was to move the Pebble container that is retrieved multiple times in src/charm.py to a cached property. There are still some other places in other Python files that retrieve the Pebble container and need to be updated. This new cached property will also be useful when we start passing the container to the single kernel library objects.

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 50.00000% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.29%. Comparing base (f5c14e1) to head (c5d5b63).
⚠️ Report is 31 commits behind head on 16/edge.

Files with missing lines	Patch %	Lines
src/charm.py	50.00%	16 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           16/edge    #1108      +/-   ##
===========================================
+ Coverage    67.23%   67.29%   +0.05%     
===========================================
  Files           19       19              
  Lines         3956     3951       -5     
  Branches       564      564              
===========================================
- Hits          2660     2659       -1     
+ Misses        1123     1119       -4     
  Partials       173      173              

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