Add `@rules_labgrid//labgrid/remote/proto` that exposes a `proto_library`

Labgrid remote booking sends messages with protobuffer messages.

As a service to downstream users, we should expose the protocol buffer service as a public proto_library for downstream users to depend on.

They are then guaranteed to get the same version of gRPC that is provided as @rules_labgrid//labgrid:pkg.