message: Add missing skip serialize
diff: @@ -107,9 +107,13 @@ struct SSServerExtConfig {
  port: u16,
  password: String,
  method: String,
  + #[serde(skip_serializing_if = "Option::is_none")]
  plugin: Option<String>,
  + #[serde(skip_serializing_if = "Option::is_none")]
  plugin_opts: Option<String>,
  + #[serde(skip_serializing_if = "Option::is_none")]
  timeout: Option<u64>,
  + #[serde(skip_serializing_if = "Option::is_none")]
  udp_timeout: Option<u64>,
  }
  