Make handler arguments optional

[Valerioageno]

Context & Description

This PR makes the (2nd, ...) handlers arguments optional.

Prev:

#[tuono_lib::handler]
async fn redirect_to_goat(_req: Request, _fetch: Client) -> Response {
    ...
}

After:
Prev:

#[tuono_lib::handler]
async fn redirect_to_goat(_req: Request) -> Response {
   ...
}