Add BatteryMonitor lib

Authored by @CrazyCoder in https://github.com/CidVonHighwind/xteink-x4-sample/pull/4
Pulled into community-sdk

libs/hardware/BatteryMonitor/include/BatteryMonitor.h

@@ -0,0 +1,30 @@
+#pragma once
+#include <cstdint>
+
+class BatteryMonitor {
+public:
+    // Optional divider multiplier parameter defaults to 2.0
+    explicit BatteryMonitor(uint8_t adcPin, float dividerMultiplier = 2.0f);
+
+    // Read voltage and return percentage (0-100)
+    uint16_t readPercentage() const;
+
+    // Read the battery voltage in millivolts (accounts for divider)
+    uint16_t readMillivolts() const;
+
+    // Read raw millivolts from ADC (doesn't account for divider)
+    uint16_t readRawMillivolts() const;
+
+    // Read the battery voltage in volts (accounts for divider)
+    double readVolts() const;
+
+    // Percentage (0-100) from a millivolt value
+    static uint16_t percentageFromMillivolts(uint16_t millivolts);
+
+    // Calibrate a raw ADC reading and return millivolts
+    static uint16_t millivoltsFromRawAdc(uint16_t adc_raw);
+
+private:
+    uint8_t _adcPin;
+    float _dividerMultiplier;
+};