Feature property morphable validation

[bentleyo]

This PR fixes the problems identified in spatie#921 (comment).

I've achieved this by doing the following:

• Requiring a #[PropertyForMorph] attribute to be added to property-morphable properties
  • By doing this it allows us to add validation messages if the concrete version of the data class cannot be determined
  • It also allows us to restrict the properties passed into morph and only run it if they are all available
• Casting properties when validating
  • This allows us to use more complex types e.g. enums or values with defaults for morphable properties without having to worry if the provided value has been cast already or not