[homekit] Allow configuring secondary services as members of a group (#13879)

[homekit] Allow configuring secondary services as members of a group (#13879)

* [homekit] allow configuring secondary services as members of a group

Required introduction of AccessoryGroup to represent the base
AccessoryInformationService for ease of configuring multiple of the
same service.

This is also "breaking" in that someone who previously had HomeKit
accessories nested directly inside of a group that was itself a
HomeKit accessory will now have those items grouped within the Home
app.

* [homekit] combine multiple readme sections on complex accessories

Signed-off-by: Cody Cutrer <cody@cutrer.us>