Migrate to Go protobuf API v2

[marcellanz]

The protobuf apiv2 has been released a few weeks ago. The protbuf project supports the apiv1 indefinitely but we don't have any obligations to do so explicitly.

refs:

• https://blog.golang.org/protobuf-apiv2
• https://github.com/protocolbuffers/protobuf-go/releases/tag/v1.20.0

tasks:

• bump to 1.4 legacy
• check any possible impacts for users apiv1

[marcellanz]

Issues
gRPC plugin: Currently the APIv2 grpc-go plugin for the protoc compiler cannot be used to generate service stubs:
golang/protobuf#903

The grpc part of the generator is in the phase to be moved to the grpc-go project:
grpc/grpc-go#3453
and as soon as its available can be used with APIv2 to gRPC services.

Non-Issues
mixing APIv1/APIv2: is possible since github.com/golang/protobuf v1.4.0 where v1.4.0 is a shim over APIv1.

[marcellanz]

a discussion about how to go (or not) forward with the new protoc plugin:
grpc/grpc-go#3669

[marcellanz]

After we changed from the v1 API with the v2 implementation, the following WIP branch uses the new protoc-gen-go-grpcand protoc-go protoc plugins. They now produce separate pb.go and grpc.pb.go files:

https://github.com/marcellanz/go-support/tree/feature/crdt_support_protobuf_v2