Change notify_callback typedef to enable the usage of member function as callback

With this change the callback function could be also a member function

src/NimBLERemoteCharacteristic.h

@@ -24,13 +24,14 @@
 #include "NimBLERemoteDescriptor.h"
 
 #include <vector>
+#include <functional>
 
 class NimBLERemoteService;
 class NimBLERemoteDescriptor;
 
 
-typedef void (*notify_callback)(NimBLERemoteCharacteristic* pBLERemoteCharacteristic,
-                                uint8_t* pData, size_t length, bool isNotify);
+typedef std::function<void (NimBLERemoteCharacteristic* pBLERemoteCharacteristic,
+                                uint8_t* pData, size_t length, bool isNotify)> notify_callback;
 
 typedef struct {
     const NimBLEUUID *uuid;