Add interfaces to make it possible to use different MQTT clients

The current concrete implementation of MqttClientConnection makes it so that there isn't a way to use a different MQTT client. A customer requested an example Greengrass Lambda function that uses this SDK to handle jobs on Greengrass - aws-samples/aws-greengrass-lambda-functions#792 - and if there was an interface I could use to replace the implementation with a Greengrass compatible one I could create the example.

I'll work on a PR for this.

Originally this issue was opened in the v2 Java Device SDK repo - https://github.com/awslabs/aws-iot-device-sdk-java-v2/issues/24