Add support for document comment in generated code

[desaikd]

Following is a solution to add support for document comments for the generated code:

Add a code generation constraint that represent document comment.
e.g.

Input Schema

type::{
    name: my_string, 
    type: string, 
    $code_gen_doc: "This is a string type"
}

Output generated code

/// This is a string type
struct MyString {
    value: String
}