Expose generation methods for buf

[neoeinstein]

This revives PR #313. With the advent of v1.0 of buf.build, the ecosystem for generating polyglot code from protobuf heavily favors the plugin model. It very easily slots into the buf generate model. This revival creates a distinct protoc-gen-prost crate to host the binary, rather than the prior method of putting the binary in the prost-build crate. This makes installation using cargo install protoc-gen-prost more convenient.

In my opinion, Prost would benefit from integrating into this model, so I wanted to revive the old protoc plugin. Even with this very basic PR (which doesn't handle options), I have confirmed that puting the binary onto the PATH and running buf generate results in precisely the Rust code I would expect over a somewhat complicated hierarchy of protobuf files.

[neoeinstein]

This has now been updated to an even smaller, minimum viable set of changes to support an external plugin executable. I've create that executable now at https://github.com/neoeinstein/protoc-gen-prost. Presuming this looks good, I'll be in a position to push up a protoc plugin crate binary.

[LucioFranco]

This is a good start! Left a question but I think we are moving in the right direction.

[LucioFranco]

Almost there! Just two more comments then I think we are good to merge.

[neoeinstein]

I believe that this is ready for a new review.