A support for 'Protobuf Deserializer' with schema registry

Description
===========
At looking into the source code for Protobuf, it seems like not possible to deserialize a protobuf message without its corresponding static message type argument.

When it comes to Avro, all things needed have been provided and it works well to deserialize an Avro message via schema registry only.

I wonder wether a support for Protobuf deserialization with schema registry is in the middle of being implemented or not planned yet.

```
class ProtobufDeserializer(object):
"""
ProtobufDeserializer decodes bytes written in the Schema Registry
Protobuf format to an object.
Args:
message_type (GeneratedProtocolMessageType): Protobuf Message type.
...
```