Add the support of PubSubHubbub protocol

Clear and concise description of the problem

PubSubHubbub (though still a working draft) is an open, simple, web-scale and decentralized pubsub protocol. In RSS channel which expects a real-time pushing, it is used to send the notification.

Suggested solution

To subscribe, the process is straightforward. Simply parse the //atom:link tag:

<feed xmlns="http://www.w3.org/2005/Atom">
   <link href="https://alampy.com/atom.xml" rel="self"/>
   <link href="https://pubsubhubbub.appspot.com/" rel="hub"/>
</feed>

Afterward, periodically send a request to register a Webhook.

Once the feed is updated, the owner will notify the hub (e.g., Google PubSubHubbub Hub), which will then forward the event to all registered Webhooks.

Alternative

No response

Additional context

• PubSubHubbub Core 0.4 -- Working Draft

Validations

• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

Start with the PubSubHubbub Core 0.4 working draft and the issue's Atom feed example, then locate the feed parsing and subscription entry points in the project. Done means recognizing atom:link elements with rel="hub", registering a webhook, and handling hub-forwarded update notifications.