Add Protocol Buffers as a schema input and output target

[sdboyer]

Thema has an increasingly robust set of lang/schema expression inputs and outputs at this point - OpenAPI, JSON Schema, Go, TypeScript (though the latter two are only outputs).

It's about time we add Protocol Buffers, especially given that it should be pretty easy given CUE's stdlib support.

...or at least, that'll help us with proto->CUE - seems there isn't a CUE->proto generator in the other direction yet. That's not optimal, but we can do the same as we did with Go for a while - given a .proto, do a runtime check that verifies it aligns with a particular Thema schema.

[sdboyer]

cc @amckinney - inching towards what we talked about!