[DPE-4257] Async replication UX Improvements

[marceloneppel]

Issue

The async replication UX between PostgreSQL and MySQL charms doesn't match.

The failover operation (promote-to-primary force=true) action is not working correctly, i.e. it's not checking if the other cluster is not reachable.

Also, it's not possible to update passwords when the consumer side is the primary cluster.

Solution

Sync the implementation with what was agreed (column G from https://docs.google.com/spreadsheets/d/1viBdDXmpkE98auo2YbFPP_RWzyHLyjXAf6oe7Fwszok/edit#gid=0).

How to test:

• Set up clusters for cross-regional async replication
• Remove or recover a cluster

Additional tests will be added in the follow-up ticket: https://warthogs.atlassian.net/browse/DPE-4538

Port of canonical/postgresql-k8s-operator#491.

[taurus-forever]

LGTM, let's merge edge and promote candidate to complete testing while waiting for feedback.

[codecov]

Codecov Report

Attention: Patch coverage is 18.00000% with 123 lines in your changes missing coverage. Please review.

Project coverage is 69.21%. Comparing base (7c306c8) to head (9ee81a1).

Files	Patch %	Lines
src/relations/async_replication.py	12.80%	109 Missing ⚠️
src/charm.py	40.00%	10 Missing and 2 partials ⚠️
src/cluster.py	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #481      +/-   ##
==========================================
- Coverage   71.06%   69.21%   -1.86%     
==========================================
  Files          11       11              
  Lines        2779     2865      +86     
  Branches      487      507      +20     
==========================================
+ Hits         1975     1983       +8     
- Misses        706      781      +75     
- Partials       98      101       +3     

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