Function get_keys_helper 

pub(crate) async fn get_keys_helper<F>(
    services: &Services,
    sender_user: Option<&UserId>,
    device_keys_input: &BTreeMap<OwnedUserId, Vec<OwnedDeviceId>>,
    allowed_signatures: F,
    include_display_names: bool,
) -> Result<Response>
where
    F: Fn(&UserId) -> bool + Send + Sync,