Event Notifications and Subscriptions (rfc3995)

[SoulDancer27]

These changes allow to create Printers and Jobs subscriptions in compliance with rfc3995. Which is an optional extension to ipp1.1.
I've also added an example script for printer subscription.
I have not checked compliance thoroughly, but tested it for printer and job subscriptions.

I'm currently working on a public printing system and have a lot of printers registered in CUPS server. This feature allows to send specified events to the designated api server and is useful to me.

I would be grateful for any response according this PR.

[andreashuetter]

This works, thanks a lot, but what still is not possible is to cancel a subscription:

    const printer = ipp.Printer(CUPS_URI);
    const response = await new Promise((resolve, reject) => {
      printer.execute('Cancel-Subscription', {
        "operation-attributes-tag": {
          "attributes-charset": "utf-8",
          "attributes-natural-language": "en",
          "printer-uri": CUPS_URI,
          "notify-subscription-id": subscriptionId
        }
      }, (err, res) => {
        if (err) {
          reject(err);
        } else {
          resolve(res);
        }
      });

Error removing subscription 56: Unknown attribute: notify-subscription-id
Error removing subscription : Unknown attribute: notify-subscription-id

Even though the attribute notify-subscription-id is defined in attributes.js:

attributes["Subscription Description"] = {
"notify-job-id": integer(1,MAX),
"notify-lease-expiration-time": integer(0,MAX),
"notify-printer-up-time": integer(1,MAX),
"notify-printer-uri": uri,
"notify-sequence-number": integer(0,MAX),
"notify-subscriber-user-name": name,
"notify-subscriber-user-uri": uri,
"notify-subscription-id": integer(1,MAX),
"subscription-uuid": uri
};

But for some reason serializer.js refuses to serialize it...