Trait for classes that have a corresponding Protobuf definition but **don't** need the ability to be directly serialized into a ByteString or ByteArray. Commonly, this interface should be used when the corresponding Protobuf message is only used in other Protobuf messages or for gRPC calls and responses, but never sent around or stored as a bytestring. E.g. an Envelope is always embedded in a Batch, and therefore doesn't need a serialization method itself because it will be indirectly serialized when the enclosing Batch is serialized.