Update deprecated jsonpb and protobuf/proto dependencies

[MartinPetkov]

The following dependencies are deprecated:

• github.com/golang/protobuf/jsonpb -> google.golang.org/protobuf/encoding/protojson
• github.com/golang/protobuf/proto -> google.golang.org/protobuf/proto

I spent some time investigating replacing these, but unfortunately it turned out to be non-trivial.

The code can be changed easily enough. However, CFT uses the *validator.Asset type from config-validator's validator.pb.go, which must be updated to use the new dependencies too. This is how far I got with attempting to do that:

• Install google.golang.org/protobuf/cmd/protoc-gen-go in the Dockerfile instead of github.com/golang/protobuf/protoc-gen-go.
• Add option go_package = "github.com/forseti-security/config-validator/pkg/api/validator"; to validator.proto.
• In the Makefile, build like so from /go/src:

protoc -I/proto -I./github.com/forseti-security/config-validator/api --go-grpc_out=. --go_out=. ./github.com/forseti-security/config-validator/api/validator.proto

• Update go.mod and go.sum (i.e. go mod tidy).

At this point I ran into an error:

$ go mod tidy
...
go: finding module for package google.golang.org/grpc/naming
github.com/forseti-security/config-validator/pkg/gcv/configs imports
	cloud.google.com/go/storage imports
	google.golang.org/api/option imports
	google.golang.org/api/internal imports
	google.golang.org/grpc/naming: module google.golang.org/grpc@latest found (v1.37.0), but does not contain package google.golang.org/grpc/naming

This seems related to micro/go-micro#2091 and duo-labs/webauthn#76. The generated validator.pb.go asserts that the minimum version of grpc-go is v1.32.0. I'm not sure if the fix here is to use an older version of protoc-gen-go or to refactor the code to not use the naming package.

As well, other repositories that depend on validator.pb.go may need to be updated to use the newer google.golang.org/protobuf/proto package, which is additional work.

At the end of the day, I'm not sure these deprecated libraries pose a large risk, but I wanted to record my investigative work for anyone who can pick this up in the future.

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