DPE-2957 async replication

[paulomach]

Issue

Need to have async replication

Solution

Add async replication library. Implementation as described by this workflow How-to draft in the discourse page

TODO:

• minimal unit test fixes
• ~discourse doc page~
• full usage of data-platform secrets
  • remove hack for secrets updates on VM (behavior differs from k8s)
• test endpoint reachability (another PR?)
• ~fence write traffic for old primary~

Integration tests and unit tests on a fup PR

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 26.75635% with 490 lines in your changes are missing coverage. Please review.

Project coverage is 59.47%. Comparing base (e7c1809) to head (0126c2f). Report is 4 commits behind head on main.

:exclamation: Current head 0126c2f differs from pull request most recent head 1a56aa8. Consider uploading reports for the commit 1a56aa8 to get more accurate results

Files	Patch %	Lines
lib/charms/mysql/v0/async_replication.py	24.20%	278 Missing and 7 partials :warning:
lib/charms/mysql/v0/mysql.py	29.82%	153 Missing and 7 partials :warning:
src/charm.py	27.11%	42 Missing and 1 partial :warning:
src/relations/mysql_provider.py	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #375 +/- ## ========================================== - Coverage 66.25% 59.47% -6.79% ========================================== Files 17 18 +1 Lines 3180 3741 +561 Branches 424 509 +85 ========================================== + Hits 2107 2225 +118 - Misses 935 1374 +439 - Partials 138 142 +4 ```

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