Implement readRSSI on Android and Darwin (#203)

example/lib/data/mock_universal_ble.dart

@@ -113,6 +113,12 @@ class MockUniversalBle extends UniversalBlePlatform {
     return 512;
   }
 
+  @override
+  Future<int> readRssi(String deviceId) async {
+    await Future.delayed(const Duration(milliseconds: 500));
+    return -50; // Mock RSSI value in dBm
+  }
+
   @override
   Future<void> setNotifiable(String deviceId, String service,
       String characteristic, BleInputProperty bleInputProperty) async {}