[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/config_BT.h

@@ -209,6 +209,7 @@ public:
     LYWSD03MMC,
     MHO_C401,
     DT24_BLE,
+    BM2,
     XMWSDJ04MMC,
     MAX,
   };