Change multi_select config validator to class (#31828)

* Move multi_select to class

* Fix serializer and add test

* Serializer should also return options

tests/helpers/test_config_validation.py

@@ -488,6 +488,14 @@ def test_multi_select():
     schema(["robban", "paulus"])
 
 
+def test_multi_select_in_serializer():
+    """Test multi_select with custom_serializer."""
+    assert cv.custom_serializer(cv.multi_select({"paulus": "Paulus"})) == {
+        "type": "multi_select",
+        "options": {"paulus": "Paulus"},
+    }
+
+
 @pytest.fixture
 def schema():
     """Create a schema used for testing deprecation."""