Add support for finding a user by external ID.

app/Http/Requests/Api/Application/Users/GetExternalUserRequest.php

@@ -0,0 +1,56 @@
+<?php
+
+namespace Pterodactyl\Http\Requests\Api\Application\Users;
+
+use Pterodactyl\Models\User;
+use Pterodactyl\Services\Acl\Api\AdminAcl;
+use Pterodactyl\Contracts\Repository\UserRepositoryInterface;
+use Pterodactyl\Exceptions\Repository\RecordNotFoundException;
+use Pterodactyl\Http\Requests\Api\Application\ApplicationApiRequest;
+
+class GetExternalUserRequest extends ApplicationApiRequest
+{
+    /**
+     * @var User
+     */
+    private $userModel;
+
+    /**
+     * @var string
+     */
+    protected $resource = AdminAcl::RESOURCE_USERS;
+
+    /**
+     * @var int
+     */
+    protected $permission = AdminAcl::READ;
+
+    /**
+     * Determine if the requested external user exists.
+     *
+     * @return bool
+     */
+    public function resourceExists(): bool
+    {
+        $repository = $this->container->make(UserRepositoryInterface::class);
+
+        try {
+            $this->userModel = $repository->findFirstWhere([
+                ['external_id', '=', $this->route()->parameter('external_id')],
+            ]);
+        } catch (RecordNotFoundException $exception) {
+            return false;
+        }
+
+        return true;
+    }
+
+    /**
+     * Return the user model for the requested external user.
+     * @return \Pterodactyl\Models\User
+     */
+    public function getUserModel(): User
+    {
+        return $this->userModel;
+    }
+}