Add ability to pass arguments to underlying client's connect method.

[yawor]

The MQTTv5 protocol no longer uses clean_session setting of the client - it's prohibited by the underlying client to even set this argument when using MQTTv5 protocol. Instead, the MQTTv5 has a clean_start argument which has to be passed to the connect method of the client.
The asyncio-mqtt Client class doesn't provide any way to pass extra arguments to underlying client's connect method. This prevents using (amongst other) the clean_start functionality with MQTTv5 protocol.
This PR proposes a way to set arguments for the underlying paho client connect method, including the clean_start one, by providing a ConnectArgs class containing all the extra args and adds an optional connect_args argument to Client constructor.

An example usage:

async def main():
    async with Client("localhost", protocol=MQTTv5, connect_args=ConnectArgs(clean_start=True)) as client:
        async with client.filtered_messages("foo/#") as messages:
            await client.subscribe("foo/#", options=SubscribeOptions(qos=1))
            async for message in messages:  # type: MQTTMessage
                print(message.topic, message.payload)

[frederikaalund]

Hi Marcin. Thanks for creating this pull request. Let me have a look. :)

It's a good idea to expose the remaining paho.Client.__init__ arguments. 👍 So far, we've done this via keyword arguments in asyncio_mqtt.Client.__init__. I see that you chose a slightly different approach with the ConnectArgs class as a layer of indirection. I suggest that we remove that layer of indirection and instead expose the remaining arguments directly. E.g.:

class Client:
    def __init__(
        self,
        hostname: str,
        port: int = 1883,
        *,
        # All the existing `paho.Client` arguments
        username: Optional[str] = None,
        password: Optional[str] = None,
        logger: Optional[logging.Logger] = None,
        client_id: Optional[str] = None,
        tls_context: Optional[ssl.SSLContext] = None,
        protocol: Optional[ProtocolVersion] = None,
        will: Optional[Will] = None,
        clean_session: Optional[bool] = None,
        transport: str = "tcp",
        # The new arguments
        keepalive: Optional[int] = None,
        bind_address: Optional[str] = None,
        bind_port: Optional[int] = None,
        clean_start: Optional[bool] = None,
        properties: Optional[Properties] = None,
    ):

What do you think? :)

[yawor]

That should be also ok. I wanted to somehow emphasize that these arguments are actually for connect in opposition to the Client instance itself.
But asyncio-mqtt API doesn't do the distinction anyway, as the client starts connection implicitly as a context manager, so maybe there's no need to complicate this. If you take this route then I think it'd be better to also copy default values for these args from the paho.
In the end, what's important is to be able to set these arguments, especially clean_start, as it can affect client logic a lot.

[frederikaalund]

Thanks, I merged it to master. :)