MINOR: sample: Add converts to parses MQTT messages

MINOR: sample: Add converts to parses MQTT messages

This patch implements a couple of converters to validate and extract data from a
MQTT (Message Queuing Telemetry Transport) message. The validation consists of a
few checks as well as "packet size" validation. The extraction can get any field
from the variable header and the payload.

This is limited to CONNECT and CONNACK packet types only. All other messages are
considered as invalid. It is not a problem for now because only the first packet
on each side can be parsed (CONNECT for the client and CONNACK for the server).

MQTT 3.1.1 and 5.0 are supported.

Reviewed and Fixed by Christopher Faulet <cfaulet@haproxy.com>