Add support to generate builders in code generation

[desaikd]

We should not expose the constructor, if we can help it. Unless we generate overloaded constructors, using a constructor means that adding a new optional field is backwards incompatible. (And if you have two optional fields with the same type, then you've got a pretty good chance of having a method signature conflict.) On the other hand, if we use builders, then the generated order doesn't matter for us or for consumers of the generated code.

I don't think it needs to be solved now, but it does need to be called out as a current limitation, and it needs to be fixed before we can say that it is ready for customers to use.

Implementation checklist for programming languages:

• Rust
• Java

Originally posted by @popematt in #95 (comment)