Support pulling proto defintions from buf registry?

[ecordell]

grpctl service set buf.build/authzed/api
# populate service name + versions + protos?

[njhale]

I took a first pass at buf support but ran into: https://github.com/bufbuild/buf/blob/0f26db21bb24b85f2608d47e3f5fbd7ccb5280ac/private/usage/usage.go#L32

then, when I tried using buf to generate its own gRPC clients, I found they don't self-host the buf registry repo: https://buf.build/bufbuild/repositories

Looks like I'm going to pull the protos for the buf registry the old fashioned way.