[8.x] Add withOnly method (#37144)

* Added in new withOnly method + tests

* Refactored function

* Update Builder.php

Co-authored-by: Taylor Otwell <taylor@laravel.com>

Author: JamesFreeman

Eloquent/Builder.php

@@ -1236,6 +1236,19 @@ public function without($relations)
         return $this;
     }
 
+    /**
+     * Set the relationships that should be eager loaded while removing any previously added eager loading specifications.
+     *
+     * @param  mixed  $relations
+     * @return $this
+     */
+    public function withOnly($relations)
+    {
+        $this->eagerLoad = [];
+
+        return $this->with($relations);
+    }
+
     /**
      * Create a new instance of the model being queried.
      *