Use git submodules for the protobufs

Although we've put in some work into making the task of updating protobuf simple, we still have the issue that the SDK has all the protobuf files committed. Besides the fact that we really shouldn't need to commit all the protobuf files, it also makes reviewing PRs more of a hassle since all the protobuf modifications are committed as well. If we can somehow update the SDK to use git sub-modules for the protobufs it would make updating the protobufs and reviewing PRs easier.