Server: Add onSubscribe() callback for characteristics.
Adds a new method to NimBLECharacteristicCallbacks that gets called when a client
changes it's subscription status.
* Remove NimBLE2902 class.
As the NimBLE2902 class usefulness was only related to callback functions that were replaced
by the NimBLECharacteristicCallbacks:onSubscribe() method this removes the NimBLE2902 class and
moves all subscription handling to NimBLECharacteristic.
* Update documents and examples to reflect this change.
* Add getSubscribedCount() to get the number of subscribed clients.
