fix: hostname validation accepts flexible IntoIterator

[JuliDi]

Right now when you only know the URL of your page but need to extract the hostname, usage becomes utterly terrible:

let host = Url::parse(&app_state.app_config.url)
    .ok()
    .and_then(|u| u.host_str().map(|h| h.to_string()));
let hostnames = host.as_deref().map(|h| [h])
let validation_response = app_state
    .turnstile_client
    .siteverify(
        SiteVerifyRequest {
            response: turnstile_response.clone(),
            ..Default::default()
        },
        hostnames.as_ref().map(|h| h.as_slice()),
    )
    .await;

This PR makes the valid_hostnames parameter more flexible (though at the cost of requiring None::<&str> to skip instead of just None, which might be good because it makes it more difficult to skip).

With this PR it becomes

let hostnames = Url::parse(&app_state.app_config.url)
    .ok()
    .and_then(|u| u.host_str().map(|h| h.to_string()))
let validation_response = app_state
    .turnstile_client
    .siteverify(
        SiteVerifyRequest {
            response: turnstile_response.clone(),
            ..Default::default()
        },
        hostnames,
    )
    .await;

[joelsa]

👍