Introduce EnumDescriptor as a way of describing EnumValueTypes

[evomimic]

Enums are an important data structure that needs to be supported in the MAP proof-of-concept. This story extends ValueDescriptorDetails to include an Enum variant and its associated EnumDescriptor.

See the MAP Property Descriptors wiki for details of Enum descriptors.

Task:

• Introduce Enum as a variant in BaseType, ValueType, and ValueDescriptorDetails enums
• Enhance test fixtures to test enum functionality
• All sweetests pass