allow path in status for ApiResponse

[ihor-rud]

This mr adds ability to use constant as status code in ApiResponse macro. I found it to be extremely convenient in my codebase. It should be easy to use the same approach in other places where literal is expected.

const OK_STATUS: u16 = 200;

#[derive(ApiResponse)]
enum MyResponse {
    #[oai(status = OK_STATUS)]
    Ok,
}