Add OpenAPI callbacks for sink-based status change events

[YadingFang]

Problem description

The CloudEvents schema is already defined, and an example (CALL_STATUS_CHANGED_EXAMPLE) for call status change notification is also provided, but this example is not referenced anywhere.

Redocly lint reports:

• CALL_STATUS_CHANGED_EXAMPLE is never used (no-unused-components)

Possible evolution

Propose to write in docs the server to consumer event delivery by using OpenAPI callbacks in POST /calls, get the sink URL from {$request.body#/sink}, and use the existing example.