Support required enum fields - `a.enum().required()`

**Is your feature request related to a problem? Please describe.**
It is currently not possible to mark an enum field as **required**.

**Describe the solution you'd like**
Support calling `.required()` as in `a.enum().required()` to mark the field as required.

**Describe alternatives you've considered**
Replacing any required `enum` with a required `string` and manually validating and restricting the values with a custom resolver. This would mean that I lose all benefit of enums. This would mean I would now have more code to manage just to implement an enum.