DPE-3264 refactor for in lib secrets

[paulomach]

Issue

• new data interfaces lib encapsulates with/without secrets logic

Solution

• Adopting changes from lib
• moved charm base unit tests to dedicated module tests/unit/test_charm_base.py

[codecov]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (990e6d9) 65.75% compared to head (a550040) 65.52%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
lib/charms/mysql/v0/tls.py	30.76%	8 Missing and 1 partial ⚠️
lib/charms/mysql/v0/mysql.py	85.71%	2 Missing and 3 partials ⚠️
src/relations/mysql.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #385      +/-   ##
==========================================
- Coverage   65.75%   65.52%   -0.23%     
==========================================
  Files          17       17              
  Lines        3130     3089      -41     
  Branches      417      406      -11     
==========================================
- Hits         2058     2024      -34     
+ Misses        940      936       -4     
+ Partials      132      129       -3     

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[taurus-forever]

LGTM! Nice!