Provide ability to use golang (v1) marshal/unmarshal under the hood

Hello 👋
We have a large protobuf repo, using golang codegen, most of which is based on the V1 go message format. We are now starting to move to using the V2 message format, and are using vtprotobuf for fast (de)serialization. However, we cannot do the migration all at once. Due to this, we have V2 generated code, that can depend on V1 format generated code during the migration. So, we need the vtproto-generated code to be compatible with the older code.

To handle such scenarios, it would be very helpful if there is an ability to substitute google.golang.org/protobuf/proto with github.com/golang/protobuf/proto. This would make our migration a lot smoother.

This could be exposed via an option/flag at codegen time.

cc @euroelessar

No files or tests are named. Start by tracing the code-generation entry point and how generation options control the protobuf runtime import; then determine how mixed v1 and v2 generated messages are represented. Done means an explicit generation option selects the v1 runtime and compatibility is covered by a reproducible test.