Function list_expired_keys_for_service 

pub(crate) fn list_expired_keys_for_service<'a>(
    relevant_periods: &[HsDirParams],
    nickname: &HsNickname,
    keymgr: &'a KeyMgr,
) -> Result<Vec<KeystoreEntry<'a>>>

Returns all the keys (as KeystoreEntry) of the service identified by nickname that are expired according to the provided HsDirParams.