description in sinkCredential

[jlurien]

Problem description

The current syntax with description under allOf was reported to cause problems with generators

        sinkCredential:
          allOf:
            - description: A sink credential provides authentication or authorization information necessary to enable delivery of events to a target.
            - $ref: "#/components/schemas/SinkCredential"

Expected behavior
In this case description could be moved to SinkCredential schema; as it is generic enough to be reused in the 2 $ref to this schema, and the allOf above can be removed.

[rartych]

We need to close RC.2 very soon, but for me the proposed correction is important for all subscription related APIs.
As we have still open PR #267 with changes in subscription template, I added the proposed changes.
@jlurien please review it.

[jlurien]

The change LGTM, but @PedroDiez will review formally the other PR on behalf of TEF

[rartych]

Closing as resolved in #267