[Compiler] Add support for additional pluggable code generation backends

[wwerner]

Currently we only have a backend for generating Java code. We should allow for plugging in additional backends, perhaps via SPI.

The following backends have been mentioned in discussions so far:

• C# (see #25)
• Java w/o Vlingo dependencies
• Rust
• Elixir
• JS/TS

I think it could also make sense to have a generic template based generator, as this would be easy to customize.