[MQTT] Possibility of allowing duplicated subscriptions using subscription identifier

MQTT5 introduced subscription identifier:
https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901117

Maybe it can be used to allow duplicated subscription. Right now, if a second subscription with the same topic filter and qos level is created, the existing subscription is returned. But if it has a different subscription identifier, maybe we should return a new, separate subscription.

A sample is also uploaded for experimenting. Right now, S1 and S2 are always pointing toward the same chunk of memory regardless of subscription properties.