Create codegen library for use in build.rs

[hchataing]

As reference https://crates.io/crates/protobuf-codegen
Create a library that can facilitate the invocation of pdlc from build.rs files.

Potential interface:

pdlc::Codegen::new()
   .input("test.pdl")
   .output("test.rs")
   .output_format(Codegen::Format::Rust)
   .run();