Port copies over values from one struct to another struct.

Unmarshal is similar to [encoding/json.Unmarshal] and parses the JSON-encoded data and stores it in the given pointer.

UnmarshalRoot is like Unmarshal, but doesn't try to call MarshalJSON on the root element.

Useful to wrap a union type to force it to use [apijson.UnmarshalJSON] since you cannot define an UnmarshalJSON function on the interface itself.