Use Protobuf as schema registry for python client

**Is your enhancement request related to a problem? Please describe.**
I was wondering if there was a possibility to use protobuf in order to auto-generate code for my pulsar client services (both producer and consumer). By reading documentation and searching on the internet I was able to find some references related to other languages (like Java or C).

**Describe the solution you'd like**
I've tried to do something like that, obviously without success.
```
import _pulsar
from pulsar import Client
from pulsar.schema import Schema

from generated_schema.test_pb2 import TodoList

class ProtobufSchema(Schema):
def __init__(self, record_cls):
super(ProtobufSchema,
self).__init__(record_cls, _pulsar.SchemaType.PROTOBUF, ,
'PROTOBUF')

def encode(self, obj):
self._validate_object_type(obj)
return obj.SerializeToString()

def decode(self, data):
return self._record_cls.FromString(data)

def test():
client = Client('pulsar://localhost:6650')
producer = client.create_producer(topic='persistent://cloudacademy/accounts/sso-configuration-updated.v1',
schema=ProtobufSchema(TodoList))

producer.send(TodoList(owner_id=1, owner_name='test'))
```

**Additional context**
For a reference, I'm expecting something like the integration with Avro, but instead with Protobuf (with autogenerated classes)

先閱讀現有的 Avro 整合以及 issue 中引用的 schema 文件，然後追蹤 Schema 如何提供 schema 類型和序列化行為。當產生的 protobuf 類別可以同時用於 producer 和 consumer，且 schema 註冊與編碼/解碼能夠一致地運作時，即表示完成。

由索引模型根據 Issue 內容生成。