build: Switch to new proto v2

[ondrej-fabry]

This PR updates Protobuf-Go to v2

Protobuf-Go

• Import path changes from github.com/golang/protobuf to google.golang.org/protobuf
  (now uses vanity import path and the actual repo is at: https://github.com/protocolbuffers/protobuf-go)

v2 is actually >=v1.20.0

• The Protobuf-Go repo actually did not increase the major version number! ⚠️
  The project authors decided to not increase major number, because of the import path changing completely
  (which is actually the important part for Go modules when doing breaking changes to an API)
• The new Protobuf-Go v2 has much better API that is easier to work with, providing true Protobuf reflection descriptors (instead of Go type reflection)
• The new interface for proto.Message has just a single method that returns the message reflect
• Generated messages no longer export fields/funcs intended for internal use 🎊 (e.g. XXX_*)
  • This makes Go docs for generated packages very clean

v1 is actually <=v1.4.0

The module github.com/golang/protobuf has switched its implementation in v1.4.0 to use the new v2 API under the hood while still providing the legacy v1 API and the v2 API allows backwards compatibility by providing conversions from v1

• github.com/golang/protobuf v1.4.0 go.mod depends on the new google.golang.org/protobuf v1.21.0
• google.golang.org/protobuf v1.20.0 go.mod depends on the old github.com/golang/protobuf v1.4.0-rc.2

gRPC

• The gRPC generator is no longer part (plugin) of protoc-gen-go and has been moved to grpc-go repo and now it is a separate generator protoc-gen-go-grpc(executed from the protoc)
• gRPC related code is now generated in a separate files - *_grpc.pb.go
• Implementators of gRPC server now must embed a generated dummy implementation of gRPC server Unimplemented*
  (this is most likely to avoid panics on when embedding nil interface)

Links for more info

• Release notes for v1.4.0: https://github.com/golang/protobuf/releases/tag/v1.4.0
• Read Golang.org blog post for info: https://blog.golang.org/protobuf-apiv2
• Protobuf-Go README for details: https://github.com/protocolbuffers/protobuf-go
• Official Protobuf Docs Go FAQ: https://developers.google.com/protocol-buffers/docs/reference/go/faq

[codecov]

Codecov Report

Merging #1691 into master will increase coverage by 0.00%.
The diff coverage is 33.33%.

@@           Coverage Diff           @@
##           master    #1691   +/-   ##
=======================================
  Coverage   58.00%   58.00%           
=======================================
  Files         370      370           
  Lines       30095    30093    -2     
=======================================
  Hits        17456    17456           
+ Misses      11197    11193    -4     
- Partials     1442     1444    +2     

Flag	Coverage Δ
#unittests	58.00% <33.33%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.