[DPE-8473] Add 'databases' to the invalid database names list

[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

Add 'databases' to the invalid database names list.

This is a fix for what was discussed on canonical/postgresql-k8s-operator#1107 (comment).

Follow-up PRs on PG VM and K8s repos:

• [DPE-8473] Handle invalid database name (databases) postgresql-operator#1190
• [DPE-8473] Handle invalid database name (databases) postgresql-k8s-operator#1108

Also, constants were added for the different substrates (VM and K8s). This allows the lib to know when it's needed to fix permissions (like for the temp storage in VM, when it's tmpfs).

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 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 46.72%. Comparing base (fb3c7f8) to head (ba6d9be).
⚠️ Report is 1 commits behind head on 16/edge.

Files with missing lines	Patch %	Lines
single_kernel_postgresql/utils/postgresql.py	80.00%	0 Missing and 1 partial ⚠️

❌ Your project status has failed because the head coverage (46.72%) is below the target coverage (70.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           16/edge      #18      +/-   ##
===========================================
+ Coverage    46.32%   46.72%   +0.39%     
===========================================
  Files            4        4              
  Lines          803      809       +6     
  Branches        94       94              
===========================================
+ Hits           372      378       +6     
  Misses         414      414              
  Partials        17       17              

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[taurus-forever]

LGTM, tnx @marceloneppel for the quick protection here!

[taurus-forever]

@7annaba3l JFYI, side-effect from charmed_databases_owner spec.
CC: @delgod