Support setting more methods for asynchronous producer

[BewareMyPower]

Currently the asynchronous producer only supports sending values, see

pulsar-client-python/pulsar/asyncio.py

Line 77 in 4629281

async def send(self, content: bytes) -> pulsar.MessageId:

and the flush method and some other properties are not implemented, see #277 (comment), we need to determine whether to implement them

The async Producer class is missing several methods that are available in the synchronous Producer class:

1. flush() - to flush buffered messages
2. producer_name() - to get the producer name
3. topic() - to get the topic name
4. last_sequence_id() - to get the last sequence ID
5. is_connected() - to check connection status