Server: Add onSubscribe() callback for characteristics. (#21)

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.

src/NimBLECharacteristic.h

@@ -110,6 +110,7 @@ public:
 
     std::string       toString();
     uint16_t          getHandle();
+    size_t            getSubscribedCount();
 
 private:
 
@@ -145,6 +146,8 @@ private:
     ble_task_data_t                *m_pTaskData;
     portMUX_TYPE                   m_valMux;
     time_t                         m_timestamp;
+
+    std::vector<std::pair<uint16_t, uint16_t>>  m_subscribedVec;
 }; // NimBLECharacteristic
 
 
@@ -181,6 +184,7 @@ public:
     virtual void onWrite(NimBLECharacteristic* pCharacteristic, ble_gap_conn_desc* desc);
     virtual void onNotify(NimBLECharacteristic* pCharacteristic);
     virtual void onStatus(NimBLECharacteristic* pCharacteristic, Status s, int code);
+    virtual void onSubscribe(NimBLECharacteristic* pCharacteristic, ble_gap_conn_desc* desc, uint16_t subValue);
 };
 
 #endif // #if defined(CONFIG_BT_NIMBLE_ROLE_PERIPHERAL)