add override utility

[ytw0728]

Hello guys! Always thanks to you guys for providing this awesome tool.

I'm not sure that it is okay to suggest my idea through this way, so if I can't, I will close this thing and reopen this as an issue instead. Please let me know :)

I want to suggest a utility to override a target schema. This utility is similar with assign, but with this I can get a type hint about the target schema.