Decouple codegen from `prost`

[LucioFranco]

prost is a great library and has already set a very good foundation for tonic. That said, we shoud look into enabling other types of codegen "drivers". This would allow us to have codegen that can create flatbuffers and json based grpc services.

• Do we want to have codegen drivers live in their own crate or feature gated behind tonic-build?

cc @cpcloud

[carllerche]

Feature gated imo

[mattsre]

I'm partial to have codegen not 100% tied into tonic-build. If it is, I believe it should be developed in a way that allows it to easily be decoupled and extracted to its own crate if the community wants. Feature-gated at first will probably be easier, and provides a simpler experience for users.

[rlabrecque]

Agreed^ It would be nice to try out rust-protobuf instead of prost for example. Or use JSON with serde_json by generating the services/models via OpenAPI, etc.

[blittable]

Agreed. Most want a rust-grpc stack that does everything that tonic does. The protobuf compiler piece is convenient until it's not and then it can be a blocker.

#30 (comment)

[LucioFranco]

I am going to drop this from 0.1 looks like we will get a prost 0.6 soon and we can release more releases to support this.