feat(enum-collector): improve extensibility of EnumTransformer

[innocenzi]

This pull request improves the extensibility of EnumTransformer.

Currently, EnumCollector is directly tied to EnumTransformer, which means that if we want to override the later, we also need to override the former.

With this pull request, the EnumCollector will check if there is a transformer that extends EnumTransformer instead of checking if EnumTransformer exists at all.

Additionally, the verification logic has been extracted to a method so it can be overridden if necessary.

The use case for this is the same as spatie/laravel-data#589

[innocenzi]

@rubenvanassche I know v3 is in the works, but would it be possible to take a quick look at that for v2? 👀

[rubenvanassche]

We'll since it is going to take a few more weeks for v3 I'll merge it!