feat: add custom logger implementation

- Implement a custom logger class
- Define logLevel property with default debug level
- Define log method to print log messages with custom format

Author: muzahidul-opti

example/lib/custom_logger.dart

@@ -0,0 +1,11 @@
+import 'package:optimizely_flutter_sdk/optimizely_flutter_sdk.dart';
+
+class CustomLogger implements OptimizelyLogger {
+  @override
+  OptimizelyLogLevel logLevel = OptimizelyLogLevel.debug;
+
+  @override
+  void log(OptimizelyLogLevel level, String message) {
+    print('[Flutter LOGGER] ${level.name}: $message');
+  }
+}