Add Rust support

[rvolosatovs]

Our protoc image(https://github.com/TheThingsIndustries/docker-protobuf) now supports Rust, so we just need to add a new build target to the Makefile

[MTRNord]

So I am kinda doing this for another project as a side module: https://github.com/Nucleon-eV/TTN-Rust

Feel free to add feedback or so. Its known that it propably wont build everywhere (the build.rs requires a specific path for the protobuf includes at the time I am writing this message)