Function im_subscribe_request_attr 

pub fn im_subscribe_request_attr(
    endpoint: u16,
    cluster: u32,
    attr: u32,
    exchange: u16,
    keep_subscriptions: bool,
) -> Result<Vec<u8>>

Build a SubscribeRequest for an attribute path (AttributeRequests, tag 3). keep_subscriptions: if true the device keeps existing subscriptions alive; if false the device cancels all prior subscriptions before creating this one.