Add support for generating enums in Rust with code generation

[desaikd]

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

Following ISL constraints will be supported with struct generation:

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

Action Items for this implementation:

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