Add support for injecting rust code into generated prost libraries

[UebelAndre]

This is a continuation of #2033 (comment)

illicitonion:

I'm imagining this taking the form roughly of a new attribute on rust_*_library along the lines of:

extra_per_package_code = {
  "my.proto.package": "path/to:extra.rs",
}

or something like that? But very open to ideas!

[UebelAndre]

This sounds like a nice idea but how would the rust_prost_library attribute make it's way into the aspect, where the crate is ultimately compiled?

[UebelAndre]

I think this will probably need to be implemented on the rust_prost_toolchain where there’s a label_keyed_string_dict of filegroups containing extra sources and label patterns to use for filtering those sources into proto files.