Enum casting option for collection_name

[chinmaypurav]

The idea is to have an enum cast for Media model.

The cast will be defined in config - default value to be string and it can be changed to any Enum::class as per the project requirements within the project namespace.

Since we now have method based casts, we can easily set the cast value based off a config file.

Do let me know if the idea is in the correct direction; if accepted by any maintainer, I can submit the PR.