feat: autogenerate protobuf service, go stubs

[TylerGillson]

This PR adds an Earthfile for autogenerating a gRPC service for OpenAI, as well as go gRPC stubs.

Some ugliness remains:

git clone https://github.com/TylerGillson/gnostic-grpc -b tyler/fix-repeated-props && \
(cd gnostic-grpc && /bin/sh plugin-creation.sh) && \

It can be cleaned up if/when the following PRs are merged:

• https://github.com/google/gnostic-grpc/pull/89
• https://github.com/google/gnostic-grpc/pull/80