Validator for protobuf's generated codes

[ijsong]

We are using protobuf intensively for various definitions of types and RPCs. Protobuf and gRPCs are great but have no built-in ways to validate messages, and it causes many boilerplate codes. Thus, we often miss many validations.

Fortunately, there are excellent plugins we can use.

• PGV (protoc-gen-validate): https://github.com/bufbuild/protoc-gen-validate
• Golang ProtoBuf Validator Compiler: https://github.com/mwitkow/go-proto-validators

How about adopting it to our code base?