[BT] Add BM2 voltage (#1495)

* [BT] Add BM2 voltage

Voltage retrieval through a BLE connection, frequency of the BLE connection is defined by `intervalcnct`

* Add doc

Credits to https://github.com/KrystianD/bm2-battery-monitor

main/ZgatewayBLEConnect.h

@@ -41,6 +41,15 @@ public:
   void publishData() override;
 };
 
+class BM2_connect : public zBLEConnect {
+  //std::vector<uint8_t> m_data;
+  void notifyCB(NimBLERemoteCharacteristic* pChar, uint8_t* pData, size_t length, bool isNotify);
+
+public:
+  BM2_connect(NimBLEAddress& addr) : zBLEConnect(addr) {}
+  void publishData() override;
+};
+
 class GENERIC_connect : public zBLEConnect {
   std::vector<uint8_t> m_data;