[Enhancement] Implement Shared Subscriptions in MQTT 5.0

Shared Subscriptions: This feature allows multiple clients to share the same subscription. When a message is published to a shared subscription, the broker delivers the message to only one of the subscribed clients, effectively load balancing the message delivery.

branch: develop_mqtt5.0
FYI:https://docs.oasis-open.org/mqtt/mqtt/v5.0/mqtt-v5.0.html

Start by checking the develop_mqtt5.0 branch and reading the MQTT 5.0 shared-subscription section linked in the issue. Trace the broker's subscription and message-delivery entry points, then define the affected tests and confirm that a message sent to a shared subscription reaches only one eligible client while ordinary subscriptions retain their behavior.

Written by the indexing model from the issue text.