Run in build.rs

There are basically two approaches to execute the code generator. The first and most evident approach is to directly execute the flutter_rust_bridge_codegen in command line.

The second approach is to integrate it into build.rs of your project. With this approach, the code generator is automatically triggered whenever you build your Rust project. For example configuration, have a look at this build.rs file. Don't forget to configure the 'build-dependency' in our cargo.toml to depend on flutter_rust_bridge_codegen = <version you use>.