Add identity arg to ModelView

Enable creating multiple views on the same model. See aminalaee#681

sqladmin/models.py

@@ -87,7 +87,7 @@ def __new__(mcls, name, bases, attrs: dict, **kwargs: Any):
             )
 
         cls.pk_columns = get_primary_keys(model)
-        cls.identity = slugify_class_name(model.__name__)
+        cls.identity = kwargs.get("identity", slugify_class_name(model.__name__))
         cls.model = model
 
         cls.name = attrs.get("name", prettify_class_name(cls.model.__name__))