Add support to use Protoboeuf as the protobuf implementation for NuevoProtobuf

[nirvdrum]

NuevoProtobuf is tightly coupled to the google-protobuf gem. We'd like to be able to use Protoboeuf instead, making it possible to switch between the two. Initially, the goal is for that integration to highlight what functionality Protoboeuf is currently missing, providing a natural way to prioritize what work to do on Protoboeuf. Longer-term, the integration would allow us to move off google-protobuf should we be unable to make necessary improvements to that gem.