aws-client: Fix example support for MQTT 3.1.1

The `MQTT_ProcessLoop()` function no longer accepts a
`timeoutMs` argument. The timeout is instead set by changing
the default value of `MQTT_RECV_POLLING_TIMEOUT_MS` via
the custom `core_mqtt_config.h` file.

Calling `MQTT_InitStatefulQoS()` after `MQTT_Init()` is now
mandatory before any other function to initialize an MQTT
context.

In CI, check that packet ACK received by checking packet type.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>