feat(config, usage): add usage-statistics-enabled option and dynamic toggling

- Introduced `usage-statistics-enabled` configuration to control in-memory usage aggregation.
- Updated API to include handlers for managing `usage-statistics-enabled` and `logging-to-file` options.
- Enhanced `watcher` to log changes to both configurations dynamically.
- Updated documentation and examples to reflect new configuration options.

config.example.yaml

@@ -21,6 +21,9 @@ debug: false
 # When true, write application logs to rotating files instead of stdout
 logging-to-file: true
 
+# When false, disable in-memory usage statistics aggregation
+usage-statistics-enabled: true
+
 # Proxy URL. Supports socks5/http/https protocols. Example: socks5://user:pass@192.168.1.1:1080/
 proxy-url: ""