Add support for prost-reflect to rust_prost_toolchain

I have a need to parse a bunch of textproto encoded files in a rust_binary target, with schemas defined in a proto_library.

To do this, I need to pull in prost-reflect as a dependency, but just as I needed to build a custom toolchain to unite prost and tonic, that rule apparently needs to include prost-reflect if desired for me to be able to do this.

Wondering if this would be possible to support.

Thanks