Integrate logging system into Flutter app with service registration and testing setup

finlog_app/fluttery/lib/logger/logger.dart

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+import 'package:fluttery/fluttery.dart';
+import 'package:logging/logging.dart' as lib;
+
+/// Abstract class for logging service.
+/// Provides methods for different log levels and configuration.
+abstract class Logger extends Service {
+  /// Logs an informational message.
+  ///
+  /// [message] is the information to log.
+  void info(String message);
+
+  /// Logs a warning message.
+  ///
+  /// [message] is the warning to log.
+  void warning(String message);
+
+  /// Logs an error message with optional error and stack trace.
+  ///
+  /// [message] is the error message to log.
+  /// [error] is the optional error object associated with this log entry.
+  /// [stackTrace] is the optional stack trace associated with this log entry.
+  void error(String message, [Object? error, StackTrace? stackTrace]);
+
+  /// Logs a debug message.
+  ///
+  /// [message] is the debug message to log.
+  void debug(String message);
+
+  /// Sets the log level for the logger.
+  ///
+  /// [level] is the new log level to set.
+  void setLogLevel(lib.Level level);
+}