Function validate_lnurl_pay 

pub async fn validate_lnurl_pay<C>(
    rest_client: &C,
    user_amount_msat: u64,
    comment: &Option<String>,
    req_data: &LnUrlPayRequestData,
    network: Network,
    validate_success_action_url: Option<bool>,
) -> Result<ValidatedCallbackResponse, LnUrlError>
where
    C: RestClient + ?Sized,

Validates invoice and performs the second and last step of LNURL-pay, as per https://github.com/lnurl/luds/blob/luds/06.md

See the parse docs for more detail on the full workflow.