Add support for generating Enums in Java

[desaikd]

Add support to generate struct based on Ion Schema. This will include supporting conversion of various enum related constraints in ISL.

Following ISL constraints will be supported with struct generation:

• valid_values (Java style enums for string/symbol values)

Action Items for this implementation:

• [ ] Define templates for enums in Java
• [ ] Define constraint mapping for valid_values in ISL to enum in Java
• [ ] Implement code generation for read and write API for enum in Java