Adding SUBSCRIPTION_DELETED as a terminationReason [event-subscription-template.yaml]

[maxl2287]

Problem description
As of now we have the current valid values of terminationReason:

    TerminationReason:
      type: string
      description: |
        - NETWORK_TERMINATED - API server stopped sending notification
        - SUBSCRIPTION_EXPIRED - Subscription expire time (optionally set by the requester) has been reached
        - MAX_EVENTS_REACHED - Maximum number of events (optionally set by the requester) has been reached
        - ACCESS_TOKEN_EXPIRED - Access Token sinkCredential (optionally set by the requester) expiration time has been reached
      enum:
        - MAX_EVENTS_REACHED
        - NETWORK_TERMINATED
        - SUBSCRIPTION_EXPIRED
        - ACCESS_TOKEN_EXPIRED

But we do not have a termination-reason when the Subscription was deleted by the User.

Possible evolution
Add SUBSCRIPTION_DELETED as a new terminationReason.

Related Issues

• Subscription-Issue1: A common way to manage lifecycle of subscriptions #153
• Consolidated issue for subscription discussion #149
• Geofencing - Adding a value in Termination Reason value enum DeviceLocation#141
• Send a CloudEvent when DELETE /subscriptions/{subscriptionId} DeviceStatus#117

[PedroDiez]

To me is fine, when discussing the model we initiallly decided to not explicitly add it. If it is consensus, no problem on adding it from our side. cc @bigludo7 , @shilpa-padgaonkar for them to be aware of this topic