Add topic-specific callback methods

[brentru]

Currently, this library returns a message callback containing a topic and payload if a subscription has been updated.

While one could filter for a specific MQTT topic within the on_message method, it is clunky.

This enhancement would add message callbacks which only trigger on a specific topic match.
Note: Topics must be subscribed to, prior to adding the callback handler.

Proposed methods:
message_callback_add(mqtt_topic, callback_method): Connect a user-defined method to a MQTT subscription topic.

• https://github.com/eclipse/paho.mqtt.python/blob/1eec03edf39128e461e6729694cf5d7c1959e5e4/src/paho/mqtt/client.py#L2182

message_callback_remove(mqtt_topic, callback_method): Remove a user-defined method from a specific mqtt topic.

• https://github.com/eclipse/paho.mqtt.python/blob/1eec03edf39128e461e6729694cf5d7c1959e5e4/src/paho/mqtt/client.py#L2199

Example Usage in Paho MQTT Python: https://github.com/eclipse/paho.mqtt.python/blob/master/examples/client_sub-multiple-callback.py#L45