* Add method to get all characteristics of a service
* Added method on NimBLEDescriptor to get the value out as a std::string
* Added methods to get things by handle where UUID could be used already.
* Added helper methods for getting a list of characteristics with a given UUID and service
* Demote the warning log for adding duplicate characteristics to a debug level log.
* Add methods to get services and characteristics using UUID + index
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.
