Adds update_consumer to nats.py

[theworksofvon]

What motivated this proposal?

Add an update_consumer method for jet stream, same as the one in the javascript library.

What is the proposed change?

pretty much a copy/paste of add_consumer just updated subject to "CONSUMER.UPDATE"

 async def update_consumer(
            self,
            stream: str,
            consumer: str,
            config: Optional[api.ConsumerUpdateConfig] = None,  # Assuming a similar class exists
            timeout: Optional[float] = None,
            **params,
        ) -> api.ConsumerInfo:
        if not timeout:
            timeout = self._timeout
        if config is None:
            config = api.ConsumerUpdateConfig()  # Assuming a similar class exists
        config = config.evolve(**params)  
        
        req = {"stream_name": stream, "consumer_name": consumer, "config": config.as_dict()}  # Adjusted key names
        req_data = json.dumps(req).encode()

        # The subject format is speculative and should be replaced with the correct format.
        subject = f"{self._prefix}.CONSUMER.UPDATE.{stream}.{consumer}"  

        resp = await self._api_request(subject, req_data, timeout=timeout)
        return api.ConsumerInfo.from_response(resp)

Who benefits from this change?

Everyone. Allows changeable items in the consumer configs to be updated.

What alternatives have you evaluated?

The only alternative is to delete and recreate a new consumer. Which is not the most suitable option.