Add support for generation of class in Java

[desaikd]

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

Following ISL constraints will be supported with struct generation:

• element
• fields
• type

Action Items for this implementation:

• [x] Define an initial workflow for code generation
• [x] Define templates for class in Java
• [x] Define constraint mapping for element, fields and type constraints in ISL to struct in Rust
• [x] Implement code generation for read and write API for struct in Rust

[desaikd]

Completed with #100