MQTT v5

[amazonKamath]

Provide support for the new MQTT5 connectivity protocol for all FreeRTOS users. MQTT v5 builds on the MQTT v3.1.1 protocol used by most devices to connect to AWS IoT Core but adding a number of new features. While the most prominent features are impacting large (Industrial) IoT devices allowing them to send multi-megabyte messages (not applicable to the typically constrained FreeRTOS MCU based devices), making the protocol integration available future-proofs FreeRTOS and will strengthen the value proposition of both FreeRTOS and IoT products.

[jnz86]

The features I would use right away are:

• Topic aliases, definitely a constrained devices issue. Taken from MQTT-SN, being able to send "1" vs "topic/some/path/12345/" would be nice.

• Reply to topic, this seems very easy. Just an optional part of the v5 header. Anything towards formalizing request / response is good imo.

• The proper ACK details and error reports. The v3 connection issues that can appear silently or only report ACK are pretty ridiculous, and should be corrected asap.

[kstribrnAmzn]

@jnz86 thank you for your comment. Knowing which features are most important will help us prioritize the order we tackle the updates required for MQTTv5.