Send a CloudEvent when DELETE /subscriptions/{subscriptionId}

[maxl2287]

Problem description
Currently there is no description about a CloudEvent when the subscription will be deleted by a User via DELETE /subscriptions/{subscriptionId}.

Possible evolution

• Update the description of `DELETE /subscriptions/{subscriptionId´ to mention that a CloudEvent will be send as well.
• Add a terminationReason like DELETE_REQUESTED

see how QoD v0.10.0 is doing it when the user requested a deletion before subscription-expired.

https://github.com/camaraproject/QualityOnDemand/blob/v0.10.0/code/API_definitions/qod-api.yaml#L918

[akoshunyadi]

The same as this in DeviceLocation camaraproject/DeviceLocation#141 , there SUBSCRIPTION_DELETED has been proposed, which I would prefer here too.
DELETE_REQUESTED suggests that there will be another event with _DELETED.

[bigludo7]

Look good for me but I check consistence with Device location (camaraproject/DeviceLocation#141) where same issue but not yet resolved.