access to parent model from withDefault closure (#23334)

Sometimes, you may wish to return default value of relations using advanced logic and condition base of parent values

This already support but not work for lazy eager loading for example

```php
// Message model
public function user()
{
        return $this->belongsTo(User::class)->withDefault(function ($user) {
            $user->name = $this->getAttribute('username');
            $user->email = $this->getAttribute('email');

            return $user;
        });
}

// single record.
$message = Message::first();
$message->user->name; // return username from message model as except.

// but i we try access to user info use lazy eager loading
$messages = Message::with('user')->get();
$messages->first()->user->name; // return null
```

This PR support pass parent model to closure to make sure the parent model available in all cases.

```php
public function user()
{
        return $this->belongsTo(User::class)->withDefault(function ($user, $parent) {
            $user->name = $parent->getAttribute('username');
            $user->email = $parent->getAttribute('email');

            return $user;
        });
}
```

thanks

src/Illuminate/Database/Eloquent/Relations/Concerns/SupportsDefaultModels.php

@@ -51,7 +51,7 @@ protected function getDefaultFor(Model $parent)
         $instance = $this->newRelatedInstanceFor($parent);
 
         if (is_callable($this->withDefault)) {
-            return call_user_func($this->withDefault, $instance) ?: $instance;
+            return call_user_func($this->withDefault, $instance, $parent) ?: $instance;
         }
 
         if (is_array($this->withDefault)) {

tests/Database/DatabaseEloquentHasOneTest.php

@@ -54,6 +54,25 @@ public function testHasOneWithDynamicDefault()
         $this->assertSame(1, $newModel->getAttribute('foreign_key'));
     }
 
+    public function testHasOneWithDynamicDefaultUseParentModel()
+    {
+        $relation = $this->getRelation()->withDefault(function ($newModel, $parentModel) {
+            $newModel->username = $parentModel->username;
+        });
+
+        $this->builder->shouldReceive('first')->once()->andReturnNull();
+
+        $newModel = new EloquentHasOneModelStub;
+
+        $this->related->shouldReceive('newInstance')->once()->andReturn($newModel);
+
+        $this->assertSame($newModel, $relation->getResults());
+
+        $this->assertSame('taylor', $newModel->username);
+
+        $this->assertSame(1, $newModel->getAttribute('foreign_key'));
+    }
+
     public function testHasOneWithArrayDefault()
     {
         $attributes = ['username' => 'taylor'];
@@ -191,6 +210,7 @@ protected function getRelation()
         $this->builder->shouldReceive('getModel')->andReturn($this->related);
         $this->parent = m::mock('Illuminate\Database\Eloquent\Model');
         $this->parent->shouldReceive('getAttribute')->with('id')->andReturn(1);
+        $this->parent->shouldReceive('getAttribute')->with('username')->andReturn('taylor');
         $this->parent->shouldReceive('getCreatedAtColumn')->andReturn('created_at');
         $this->parent->shouldReceive('getUpdatedAtColumn')->andReturn('updated_at');
         $this->parent->shouldReceive('newQueryWithoutScopes')->andReturn($this->builder);