Function restmail_client::find_email

pub fn find_email<F>(
    email: &str, 
    predicate: F, 
    max_tries: u8
) -> Result<EmailJson, RestmailClientError> where
    F: Fn(&EmailJson) -> bool, 

For a given restmail email, find the first email that satisfies the given predicate. If no email is found, this function sleeps for a few seconds then tries again, up to max_tries times.