Add support for capturing unknown fields in protobuf

[Egorand]

It's my understanding that during proto deserialization the fields that don't match any @SerialId tags get discarded. It would be useful to allow capturing these fields in a ByteArray so that they can be persisted and deserialized at a later point in time when the schema gets updated.

[robxyy]

Are there any updates? Unknown fields are useful for synchronizing data.