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[bot]]

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 :warning:
lib/charms/mysql/v0/mysql.py	85.71%	2 Missing and 3 partials :warning:
src/relations/mysql.py	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ 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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