Add support for MQTT v5.0

[dborovcanin]

Is your feature request related to a problem? Please describe.

MQTT 5.0 is not mature enough to consider updating mGate to work with it.

Describe the feature you are requesting, as well as the possible use case(s) for it.

Some limitations of MQTT v3.1.1, such as lack of user properties requested here and lack of proper reason code for PUB and SUB packets (during authorization, we can forbid the operation, but we can't notify client about the result since in v3.1.1 reason is available only for CONNACK and SUBACK) are motivation to switch to MQTT v5.0

Indicate the importance of this feature to you.

Must-have

Anything else?

N/A