[DPE-3684][DPE-6020] Reinitialise raft

[dragomirp]

Syncobj RAFT implementation, used as a standalone DCS for Patroni, cannot elect a leader if the cluster loses quorum and becomes read only. This will prevent Patroni from automatically switching over, even in cases where sync_standbys are available in the cluster and could take over as primary.

The PR adds logic to detect when the RAFT cluster becomes read only and updates promote-to-primary action to be able to trigger a RAFT re-initialisation.

Contains changes from #663

[codecov]

Codecov Report

Attention: Patch coverage is 76.25571% with 52 lines in your changes missing coverage. Please review.

Project coverage is 71.85%. Comparing base (58449da) to head (bf025dc).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/charm.py	76.76%	19 Missing and 14 partials ⚠️
src/cluster.py	75.00%	17 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #611      +/-   ##
==========================================
+ Coverage   71.38%   71.85%   +0.47%     
==========================================
  Files          15       15              
  Lines        3253     3464     +211     
  Branches      480      535      +55     
==========================================
+ Hits         2322     2489     +167     
- Misses        812      844      +32     
- Partials      119      131      +12     

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