feat: add abillity to use pre-built protos

[micahcc]

• prost-build supports compile_fds https://docs.rs/prost-build/latest/prost_build/fn.compile_fds.html
• this is helpful for cases like protox: https://github.com/andrewhickman/protox which replaces the protoc executable

Usage example of protox:

let file_descriptors = protox::compile(["root.proto"], ["."]).unwrap();
prost_reflect_build::compile_fds(file_descriptors).unwrap();