Derive Macro Encode 

#[derive(Encode)]
{
    // Attributes available to this derive:
    #[field]
    #[variant]
}

Derives the Encode trait for a struct or enum.

For structs, encodes each field in declaration order. Field attributes control encoding behavior (varint, optional, length-prefixed, rest).

For enums, writes a VarInt discriminant followed by the variant’s fields. Discriminants are sequential from 0 unless overridden with #[variant(id)].

For packet structs, use #[packet(id = N)] instead — it generates Encode, Decode, EncodedSize, and PACKET_ID all at once.