API to generate .rs files for ttrpc from protobuf

API to generate .rs files to be used e. g. from build.rs.

Example code:

```rust use ttrpccodegen::Codegen; use ttrpccodegen::Customize;

fn main() { Codegen::new() .outdir("protocols/sync") .inputs(&protos) .include("protocols/protos") .rustprotobuf() .customize(Customize { ..Default::default() }) .run() .expect("Gen code failed."); }

```

And in Cargo.toml:

[build-dependencies] ttrpc-codegen = "0.3"

The alternative is to use protoc-rust crate, which relies on protoc command to parse descriptors. Both crates should produce the same result, otherwise please file a bug report.