Implement Codegen

[galvesribeiro]

The first release was meant to proof that all the concept works properly and that we can have proper http endpoints exposed on Orleans. However, it was implemented with a Reflection all over the place at startup.

I would like to remove all that reflection and implement a Roslyn-based codegen so we can pre-emptively generate all grain invokers based on routes and type handlers. That would speed up the startup.