Stabilize the ProtoRegistry API once go-protobuf's api-v2 lands

[jmillikin-stripe]

When the new proto.Message interface lands in a stable upstream release (golang/protobuf#364), we can use it for reflective operations instead of making assumptions about the generated struct layout. This is a hard requirement for a stable ProtoRegistry interface.

[jmillikin-stripe]

Looks like the new API has been released: https://blog.golang.org/a-new-go-api-for-protocol-buffers

[dilyevsky]

It seems like in order to be able to use the whole library needs to be converted to use new api and its https://google.golang.org/protobuf/types/dynamicpb