denial from verification as get request

[Kongaloosh]

Subscriptions MAY be validated by the Hubs who may require more details to accept or refuse a subscription. The Hub MAY also check with the publisher whether the subscription should be accepted.

If (and when) the subscription is accepted, the hub MUST perform the verification of intent of the subscriber.

If (and when) the subscription is denied, the hub MUST inform the subscriber by sending an HTTP [RFC7231] (or HTTPS [RFC2818]) GET request to the subscriber's callback URL as given in the subscription request. This request has the following query string arguments appended (format described in Section 4 of [URL]):

hub.mode
REQUIRED. The literal string "denied".
hub.topic
REQUIRED. The topic URL given in the corresponding subscription request.
hub.reason
OPTIONAL. The hub may include a reason for which the subscription has been denied.
The subscription MAY be denied by the hub at any point (even if it was previously accepted). The Subscriber SHOULD then consider that the subscription is not possible anymore.