[DOP-19902] Add MySQL API schema

[IlyasDevelopment]

Change Summary

• Added Pydantic schemas for MySQL connection
• Added Pydantic schemas for MySQL source parameters and target parameters
• Added unit tests for creating & updating MySQL connection and target
• Updated README to include references to MySQL

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.52%. Comparing base (9a03779) to head (e843320).
Report is 78 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #126      +/-   ##
===========================================
+ Coverage    87.39%   87.52%   +0.12%     
===========================================
  Files          142      143       +1     
  Lines         3308     3342      +34     
  Branches       251      251              
===========================================
+ Hits          2891     2925      +34     
  Misses         355      355              
  Partials        62       62              

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