Cannot specify subscription identifier when sending a SUBSCRIBE packet

Based on my cursory glance, it looks like specifying a subscription identifier is not supported for the SUBSCRIBE packet. At the very least, it's not part of the `ISubscribePacket` type definition. I currently don't have time to fix it but I'm posting the issue to document this. I may come back with a PR at a later point.

If someone has the time to investigate this and make a PR, feel free.