* for (Challenge challenge : response.challenges()) {
 *   // If this is preemptive auth, use a preemptive credential.
 *   if (challenge.scheme().equalsIgnoreCase("OkHttp-Preemptive")) {
 *     return response.request().newBuilder()
 *         .header("Proxy-Authorization", "secret")
 *         .build();
 *   }
 * }
 * return null; // Didn't find a preemptive auth scheme.
 * ```
 *
 * ## Reactive Authentication
 *