Define a test runner for roundtrip generated code tests

[desaikd]

Define a procedural macro or build script that generates tests for generated code.
This way we can use reference to the generated code even without the need to generate it first.

[desaikd]

Defining a test runner requires following changes in the current crate structure:

• Generate code in a single module instead of multiple files/modules inside ion_data_model.
• Create a separate directory for storing generated code in gradle project or crate.
• The new directory should include some test cases for input schema and Ion input.
• Write tests in ion-cli that runs the code generator and generates code into newly created directory that has java and rust projects.

[desaikd]

Completed with #94, #95