Massively simplify API binding logic

Changes the API internals to use normal Laravel binding which automatically supports nested-models and can determine their relationships. This removes a lot of confusingly complex internal logic and replaces it with standard Laravel code.

This also removes a deprecated "getModel" method and fully replaces it with a "parameter" method that does stricter type-checking.

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

@@ -2,6 +2,7 @@
 
 namespace Pterodactyl\Http\Controllers\Api\Application\Users;
 
+use Pterodactyl\Models\User;
 use Pterodactyl\Transformers\Api\Application\UserTransformer;
 use Pterodactyl\Http\Controllers\Api\Application\ApplicationApiController;
 use Pterodactyl\Http\Requests\Api\Application\Users\GetExternalUserRequest;
@@ -11,9 +12,11 @@ class ExternalUserController extends ApplicationApiController
     /**
      * Retrieve a specific user from the database using their external ID.
      */
-    public function index(GetExternalUserRequest $request): array
+    public function index(GetExternalUserRequest $request, string $external_id): array
     {
-        return $this->fractal->item($request->getUserModel())
+        $user = User::query()->where('external_id', $external_id)->firstOrFail();
+
+        return $this->fractal->item($user)
             ->transformWith($this->getTransformer(UserTransformer::class))
             ->toArray();
     }