Refactoring publication to support zero-copy serialization

[ryan-summers]

This PR is designed to allow an end user to provide a Publication message with a yet-to-be-serialized payload. This then allows them to serialize their payload directly into the MQTT TX buffer, which eliminates the need for them serializing and then copying it over into the MQTT payload.

Example:

client.publish(Publication::new_deferred(|buf| settings.get(key, buf)));