Provide a method in prost_build::Config to get generated code straightly

[Hexilee]

I want to generate code in a mod block by procedural macro. So I need a method to get generated code straightly like this:

fn compile<P>(&mut self, protos: &[P], includes: &[P]) -> io::Result<String>
        where
            P: AsRef<Path> {
        ...
}

and return code like this:

struct HelloRequest {
    ...
}

struct HelloReply {
    ...
}

// generated custom service
pub mod server {
    ...
}

[Hexilee]

Is a PR welcome?