*     .header("Proxy-Authorization", credential)
 *     .build();
 * ```
 *
 * The proxy authenticator may implement preemptive authentication, reactive authentication, or
 * both.
 *
 * Applications may configure OkHttp with an authenticator for origin servers, or proxy servers,
 * or both.
 *
 * ## Authentication Retries
 *
 * If your authentication may be flaky and requires retries you should apply some policy