Add `has_cos_relation` to the MySQLCharmBase class

[shayancanonical]

Issue

• We added a has_cos_relation property in a branch in the k8s charm. It seems like this property would be good to have in the vm charm as well as the k8s charm
• Sometimes, we pass the current_event (event of a handler) down a chain of methods before it gets used

Solution

• Add has_cos_relation property to the MySQLBaseCharm class
• Introduce a self.current_event field (initialized to None) that can be set on the charm class by an event and then used in helper methods

[codecov[bot]]

Codecov Report

Merging #289 (57fdad8) into main (0e6bcad) will decrease coverage by 0.03%. The diff coverage is 55.55%.

@@            Coverage Diff             @@
##             main     #289      +/-   ##
==========================================
- Coverage   62.38%   62.36%   -0.03%     
==========================================
  Files          14       14              
  Lines        2680     2686       +6     
  Branches      343      344       +1     
==========================================
+ Hits         1672     1675       +3     
- Misses        900      903       +3     
  Partials      108      108              

Files Changed	Coverage Δ
lib/charms/mysql/v0/mysql.py	70.03% <55.55%> (-0.15%)	:arrow_down:

[shayancanonical]

Thanks for the reviews!