Refactor 2904 descriptor

* General code cleanup.
* Added method `NimBLECharacteristic::create2904` which will specifically create a CPF(0x2904) descriptor.

examples/Advanced/NimBLE_Server/main/main.cpp

@@ -195,7 +195,7 @@ void app_main(void) {
      *  and sizes. However we must cast the returned reference to the correct type as the method
      *  only returns a pointer to the base NimBLEDescriptor class.
      */
-    NimBLE2904* pBeef2904 = (NimBLE2904*)pBeefCharacteristic->createDescriptor("2904");
+    NimBLE2904* pBeef2904 = pBeefCharacteristic->create2904();
     pBeef2904->setFormat(NimBLE2904::FORMAT_UTF8);
     pBeef2904->setCallbacks(&dscCallbacks);