[Feature] Provide Groups from z2m to HomeKit
First, i want to say, i love this plugin. It works perfect for me.

But i would like the request a feature. I already read #166 but i would like to start a new "issue" for that.

**What would be cool**
All groups from zigbee2mqtt are exposed in HomeKit.
Different to #166 it is not required to subsequently hide all devices assigned to a group. Sometimes it is still necessary to keep each device next to the group. Nevertheless, excluding unwanted devices is already possible.
Just adding a new "device" to HomeKit would be great.

**Why this would be cool**
Because sending a command to a group needs just one zigbee transmission. The devices assigned to a group react simultaneously and thats beautiful. Especially when it comes to range problems, a group is much more reliable than sending 8 commands to 8 lightbulbs. 

**How this could be accomplished**
 There should be no differences to exposing a device, because the mqtt topics are the friendly name of the group (assigned in zigbee2mqtt) and are exposed by in its seperate mqtt topic, just like the devices.
A hurdle is, "zigbee2mqtt/bridge/groups" does not provide any functions of its members. So it becomes necessary to assign a device to the group in Homebridge.
Otherwise, i think an auto-detect of groups is not necessary at all. With adding "groups" to the config.json and enumerate each friendly name of the group you want to use and assign a device group (lightbulb RGB or whatever) should do the job.

Thanks for attention :)