let's add side-by-side with "official" protobuf marshaling

[DmitriyMV]

let's add side-by-side with "official" protobuf marshaling
Originally posted by @smira in #1 (review)

Task

We should ensure that our encoder/decoder works with binary data and structs produced by official protobuf compiler. For that each test should basically contain the following.

1. Marshal Go structs to binary form using our own Marshaller
2. Unmarshal binary form into generated Go structs compiled by protobuf from proto messages using proto.Unmarshal
3. Ensure that fields in two structs match.
4. Encode resulting generated Go structs back to binary form using proto.Marshal
5. Unmarshal binary form into original Go structs using our own Unmarshaller.
6. Ensure that resulting struct match the original one from the stage 1.

The following test cases should be implemented:

• Messages with simple fields.
• Messages with repeated simple fields.
• Messages with maps of simple fields.
• Messages containing a map where the key is a simple type and the value is a message that contains variations of the above.
• Complex messages with inner messages which contain either maps or repeated fields or both in various forms.

anyof and to lesser extent enum are not supported for now.

Questions

• Does kres support code generation for protobuf messages that are not attached to GRPC methods?