Add Granular Model Validation

[ThomasLaPiana]

Code Changes

The goal here is to add custom validation logic directly to each struct using a new trait. This gives the ability to surface much more helpful error messages!

The new implementation will see .validate() added as a method to existing models, which returns either () or the new ValidationError result

• add a new Trait or validation
• add test scaffolding + tests for the new validation logic
• implement the trait and add validation logic to the following models:
  • Task
  • VersionRequirements
  • Templates