[Server] Add ability to create static attributes.

This allows creating instances of services/characteristics and decriptors without
needing to call the parent createX() methods. This allows creation of the attibutes
within a class or other structure and the attributes can be linked to the parent by
calling the parent addX() method.

Additonally this increases the backward compatibility with the original bluedroid library
to further support user migration.

src/NimBLEService.cpp

@@ -246,6 +246,11 @@ NimBLECharacteristic* NimBLEService::createCharacteristic(const NimBLEUUID &uuid
 } // createCharacteristic
 
 
+void NimBLEService::addCharacteristic(NimBLECharacteristic* pCharacteristic) {
+    pCharacteristic->setService(this);
+    m_chrVec.push_back(pCharacteristic);
+}
+
 /**
  * @brief Get a pointer to the characteristic object with the specified UUID.
  * @param [in] uuid The UUID of the characteristic.