Add alias for empty input/output payload.

Hi! I would like to suggest adding a new type alias for empty input/outputs. Right now, if a route does not accept any input and/or output, it should be defined as a pointer to an empty structure. In my project I have a separate type alias for this as it is more convenient and clearer, so I thought it might be nice to have it built-in (e.g. ```huma.Empty```, ```huma.EmptyPayload```).

If you think this can be helpful, I'd like to contribute.