Add TypeDecorator.cache_ok (#124)

* Add TypeDecorator.cache_ok

* Convert to ClassVar

Author: kasium

sqlalchemy-stubs/sql/type_api.pyi

@@ -1,4 +1,5 @@
 from typing import Any
+from typing import ClassVar
 from typing import Generic
 from typing import Mapping
 from typing import Optional
@@ -125,6 +126,7 @@ class NativeForEmulated:
 class TypeDecorator(SchemaEventTarget, TypeEngine[_T]):
     __visit_name__: str = ...
     impl: Any = ...
+    cache_ok: ClassVar[Optional[bool]] = ...
     def __init__(self, *args: Any, **kwargs: Any) -> None: ...
     coerce_to_is_types: Tuple[Type[Any], ...] = ...
     class Comparator(TypeEngine.Comparator[_TE]):