MCC-81949 Field_type and validator checks w/ specs

- class FieldTypeChecker contains the rules from Andrey's documentation within the singleton members
- used pattern to pass in the DescriptorValidator.self in order to access the base class add_error method in FieldTypeChecker. Can't think of a simpler way.
- Errors are: 1) bad field_type name 2) bad validator name 3) validator for field type is not allowed