main: Support subscription mode

Import and adjust MQTT_EVT_PUBLISH case handler from:
repo: https://github.com/zephyrproject-rtos/zephyr


cid: 730acbd6ed85a5ee30d66f305cfc159b24643468
path: samples/net/cloud/google_iot_mqtt/src/protocol.c

Add subscriber() routine that will subscribe to the
topic and then ping the broker and handle incoming
MQTT events/messages.

Signed-off-by: Zbigniew Bodek <zbigniew.bodek@huawei.com>