Feature request: option for JsonFormat.Printer to fail if unknown fields are encountered

Currently, com.google.protobuf.util.JsonFormat.Printer silently ignores unknown fields. It would be nice to have a proactive safety check that would fail if the passed in MessageOrBuilder had unknown fields. Ideally, this would apply recursively to any nested messages, and also account for unknown fields in well known message types as well.

While encountering an unknown fields at this layer is most likely an application-level error (that is, the application probably deserialized some binary data without checking if there were any unknown fields, and then later on decided to serialize it as JSON), it would be nice to have a built-in option for this.