Creating a route that makes a call from the server to an external service which is firewalled, with data sent back to the caller (like a ReactJS component)

SearchController.php

<?php

namespace Drupal\harlib_search\Controller;

use Drupal\Core\Controller\ControllerBase;
use GuzzleHttp\Client;
use Psr\Log\LoggerInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\HttpFoundation\RequestStack;
use GuzzleHttp\Exception\RequestException;
use Symfony\Component\HttpFoundation\Response;

/**
 * Class SearchController.
 *
 * @package Drupal\harlib_search\Controller
 */
class SearchController extends ControllerBase {

  /**
   * The base url to do searches against.
   *
   * @var string
   */
  protected $baseSearchUrl = "http://YOUR_ENDPOINT_HERE";

  /**
   * Symfony\Component\HttpFoundation\RequestStack definition.
   *
   * @var \Symfony\Component\HttpFoundation\RequestStack
   */
  protected $requestStack;

  /**
   * GuzzleHttp\Client definition.
   *
   * @var \GuzzleHttp\Client
   */
  protected $httpClient;

  /**
   * Psr\Log\LoggerInterface definition.
   *
   * @var \Psr\Log\LoggerInterface
   */
  protected $logger;

  /**
   * Constructor for our class.
   */
  public function __construct(RequestStack $request_stack, Client $http_client, LoggerInterface $logger) {
    $this->requestStack = $request_stack;
    $this->httpClient = $http_client;
    $this->logger = $logger;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('request_stack'),
      $container->get('http_client'),
      $container->get('logger.factory')->get('harlib_search')
    );
  }

  /**
   * Controller action. Show the ReactJS component.
   */
  public function index() {
    return [
      '#theme' => 'harlib_search',
    ];
  }

  /**
   * Fetch results from Hollis.
   *
   * @return \Symfony\Component\HttpFoundation\Response
   *   A Symfony response object.
   */
  public function results() {
    $data = '';
    $uri = $this->baseSearchUrl . $this->requestStack->getCurrentRequest()->getQueryString();

    try {
      $response = $this->httpClient->get($uri, ['headers' => ['Accept' => 'text/xml']]);
      $data = (string) $response->getBody();
    }
    catch (RequestException $e) {
      $this->logger->error("Error returned from HOLLIS search, error was @error", ['@error' => $e->getMessage()]);
    }

    return new Response($data, 200);
  }

}