Create codegen library for use in build.rs

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();
```