Rework secrets

[marceloneppel]

Issue

• Secrets like passwords and TLS certificates should be moved to Juju secrets in the future.
• Currently, passwords are being set and retrieved from app relation data in multiple places, what can lead to a big refactor later (even more when we add TLS certificates to the charm).

Solution

• Move secret implementation logic to specific functions that can be easily refactored in the future to use Juju secrets instead of app relation data.

Context

• Unit peer relation property was also added for future use cases.
• Some constants were created to avoid the same strings in multiple places.
• This was already done in a very similar way in Rework secrets postgresql-k8s-operator#27.
• Checking again Will's comment in the other PR, I saw that it makes sense for the other relation data that the charm uses (like list of endpoints). So an issue was created to move all operations related to the peer relation to another file.

Testing

• Additional unit tests were added.
• Tested using integration tests (to ensure the new implementation works the same way).

Release Notes

• Add specific functions to set and get secrets.

[codecov-commenter]

Codecov Report

Merging #23 (f15a1b7) into main (ceeba02) will increase coverage by 0.15%.
The diff coverage is 74.54%.

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
+ Coverage   60.11%   60.27%   +0.15%     
==========================================
  Files           6        6              
  Lines         692      730      +38     
  Branches      103      113      +10     
==========================================
+ Hits          416      440      +24     
- Misses        256      264       +8     
- Partials       20       26       +6     

Impacted Files	Coverage Δ
src/charm.py	52.77% <72.54%> (+0.59%)	⬆️
src/cluster.py	54.81% <100.00%> (+0.33%)	⬆️
src/constants.py	100.00% <100.00%> (ø)

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[paulomach]

lgtm