Fix Callables for Extenders (#2423)

- Standardize signatures and variable names for extenders that take callbacks
- Adjust model extender docblock to clarify that default calue can't be an invokable class.
- Make invokable classes provided to Model->relationship
- Add integration tests to ensure Model->relationship and User->groupProcessor extenders accept callbacks
- Extract code for wrapping callbacks into central util

tests/integration/extenders/ModelTest.php

@@ -134,6 +134,23 @@ class ModelTest extends TestCase
         $this->assertEquals([], $user->customRelation()->get()->toArray());
     }
 
+    /**
+     * @test
+     */
+    public function custom_relationship_can_be_invokable_class()
+    {
+        $this->extend(
+            (new Extend\Model(User::class))
+                ->relationship('customRelation', CustomRelationClass::class)
+        );
+
+        $this->prepDB();
+
+        $user = User::find(1);
+
+        $this->assertEquals([], $user->customRelation()->get()->toArray());
+    }
+
     /**
      * @test
      */
@@ -423,3 +440,11 @@ class ModelTestCustomPost extends AbstractEventPost
      */
     public static $type = 'customPost';
 }
+
+class CustomRelationClass
+{
+    public function __invoke(User $user)
+    {
+        return $user->hasMany(Discussion::class, 'user_id');
+    }
+}