Add support for finding a user by external ID.

app/Http/Controllers/Api/Application/Users/ExternalUserController.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace Pterodactyl\Http\Controllers\Api\Application\Users;
+
+use Pterodactyl\Transformers\Api\Application\UserTransformer;
+use Pterodactyl\Http\Controllers\Api\Application\ApplicationApiController;
+use Pterodactyl\Http\Requests\Api\Application\Users\GetExternalUserRequest;
+
+class ExternalUserController extends ApplicationApiController
+{
+    /**
+     * Retrieve a specific user from the database using their external ID.
+     *
+     * @param \Pterodactyl\Http\Requests\Api\Application\Users\GetExternalUserRequest $request
+     * @return array
+     */
+    public function index(GetExternalUserRequest $request): array
+    {
+        return $this->fractal->item($request->getUserModel())
+            ->transformWith($this->getTransformer(UserTransformer::class))
+            ->toArray();
+    }
+}