[12.x] Ensure custom validation messages work for AnyOf, Can and Enum

[jackbayliss]

Currently for the the Enum validation rule Can and anyOf to do a custom validation message you need use the fully qualified class name, ie:

 'school.Illuminate\Validation\Rules\Enum' => 'Please enter valid school'

This PR makes it so you can do the short (normal?) string version ie:

 'school.enum' => 'Please enter valid school'

Which is inline with other rules - such as:

    'forename.required' => 'Forename is required',
    'surname.required' => 'Surname is required',
    'school.enum'=> 'Please select a valid school',

I've added tests for all affected rules to prove it works for both the fqcn and short hand which should help prevent regression.

I don't believe this is a breaking change- which is why I did it this way - can target 13.x - open to feedback!

Thanks

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