add BME680 IAQ reading. Range is from 0 (clean) - 500 (extremely polluted)

src/modules/Telemetry/Sensor/BME680Sensor.cpp

@@ -57,6 +57,7 @@ bool BME680Sensor::getMetrics(meshtastic_Telemetry *measurement)
     measurement->variant.environment_metrics.barometric_pressure = bme680.getData(BSEC_OUTPUT_RAW_PRESSURE).signal / 100.0F;
     measurement->variant.environment_metrics.gas_resistance = bme680.getData(BSEC_OUTPUT_RAW_GAS).signal / 1000.0;
     // Check if we need to save state to filesystem (every STATE_SAVE_PERIOD ms)
+    measurement->variant.environment_metrics.iaq = bme680.getData(BSEC_OUTPUT_IAQ).signal;
     updateState();
     return true;
 }