add top-level enum attribute `id` about deku HOT 4 CLOSED

#[derive(Debug, PartialEq, DekuRead, DekuWrite)]
struct AsterixPacket {
    #[deku(bytes = "1", endian = "big")]
    category: u8,
    #[deku(bytes = "2", endian = "big")]
    length: u16,
    #[deku(ctx = "*category")]
    messages: Vec<Message>,
}

#[derive(Debug, PartialEq, DekuRead, DekuWrite)]
#[deku(ctx = "category: u8", id = "category")]
enum Message {
    #[deku(id = "48")]
    Cat48(Cat48),
}