Update remote characteristic value from a notification

* Add NimBLERemoteCharacteristic::getValue(time_t *timestamp = nullptr) to get the latest remote characteristic and (optionally) it's timestamp.

* Added a timestamp to NimBLEAdvertisedDevice for the moment a device was scanned

src/NimBLEAdvertisedDevice.cpp

@@ -546,6 +546,11 @@ uint8_t NimBLEAdvertisedDevice::getAddressType() {
 }
 
 
+time_t NimBLEAdvertisedDevice::getTimestamp() {
+    return m_timestamp;
+}
+
+
 void NimBLEAdvertisedDevice::setAddressType(uint8_t type) {
     m_addressType = type;
 }