Added support for String enumerations

**The issue**
Currently enumerations cannot be validated using the validation api.
In addition to that the default generator doesn't leave a hint that only specified values can be used for this entry maing it almost impossible for a annotation aware generator to generate a valid request.

**The Fix**
If no Regex pattern is specified the generator will add a regex pattern for all enumerations.
Due to this change all String enumeration values can be easyly checked by looking at the annotation.