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.
* Use critical sections to access characteristic/descriptor value data.
* Remove unnecessary code
* Create characteristic semaphore only if needed for indications.
* Fix advertising when not broadcasting a service.
This allows NimBLE options in menuconfig to reduce code size based on
the roles selected (scan/advertising/central/peripheral).
Significant code space can be saved by removing unnecessary roles for the application.
