MGMT-112 Improve the message validators

[dbhynds]

Summary of Changes

This refactors our MessageValidators and improves testing on them. It abstracts validation common to all validators into a base class, leaving the individual implementations to test the message-type-specific rules.

I also added extensive unit tests for the validators.

Testing

Here are the changes passing in our platform: https://gitlab.com/packback/questions/-/pipelines/692545051

• [x] I have added automated tests for my changes
• [x] I ran composer test before opening this PR
• [x] I ran composer lint-fix before opening this PR

[dbhynds]

Alright, so after further research, I decided not to switch over to pint. As nice as it would be, it just isn't as performant on a large-scale app. Running it on the App API take 5-10 minutes, compared to <45s with our old linter. It's not worth the performance hit, so I switched back to the old linter.

[dbhynds]

New pipeline with latest changes: https://gitlab.com/packback/questions/-/pipelines/694475065