zigbee: Add Groupcast (Multicast) support

As described, this MR adds support for data groupcasting transmission mode to the Zigbee application support layer (APS).

A groupcast is essentially a multicast transmission in which the destination is all the end devices in a group. This is particularly useful in scenarios where a device needs to send the same data to multiple endpoints simultaneously, such as in lighting control systems or sensor networks.

• Documentation has been extensively updated
• Zigbee concept stack figure was updated
• The NWK required the most changes to support "multicast" route discovery
• Primitives to add and remove groups from devices were added to the APS
• An example was integrated into to zigbee-aps-data.cc to show usage

Question, comments are welcome.