Add back grpc c++ support

[bplotnick]

We used to be able to pass in use_grpc_plugin to pgv_cc_proto_library until #92. This is useful for c++ applications talking to Envoy.

You can kind of see a use-case in this PR that i let fall by the wayside: https://github.com/envoyproxy/envoy/pull/3857/files

It'd be nice to have this back in some way.

[rodaine]

@sesmith177 is this related to your bazel changes, perhaps?

[sesmith177]

Yep, it looks like use_grpc_plugin was handled in the protobuf.bzl here, and when we broke the dependency on that file, the functionality did not make it over