Handle "required" fields in protobuf compatibility layer

[EricWittmann]

I am testing BACKWARD and BACKWARD_TRANSITIVE compatibility modes for PROTOBUF schemas (artifacts) and it looks like it is not working properly.

This is my use case:

1. Register 1st version of protobuf artifact:

syntax = "proto2";

message ProtoSchema {
  required string message = 1;
  required int64 time = 2;
}

2. Switch on the BACKWARD compatibility mode on that artifact
3. Register 2nd version of protobuf artifact which is not backward compatible with 1st version (a required field is added):

syntax = "proto2";

message ProtoSchema {
  required string message = 1;
  required int64 time = 2;
  required string code = 3;
}

4. Artifact gets registered without the error on compatibility rule check.

[EricWittmann]

Fixed in #2439