Add callback definition for call status change events

[YadingFang]

What type of PR is this?

Add one of the following kinds:

• enhancement/feature

What this PR does / why we need it:

This PR adds ClickToDial call status change notifications using OpenAPI 3 callbacks under POST /calls, resolving the callback URL from the consumer-provided sink field ({$request.body#/sink}).

It also references the existing CALL_STATUS_CHANGED_EXAMPLE CloudEvents payload in the callback request body, addressing the Redocly lint warning about an unused example component.

This change formalizes the existing sink-based notification mechanism in the OpenAPI spec using callbacks.

Which issue(s) this PR fixes:

Fixes #62

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ ACTION	actionlint	2		0	0.0s
✅ API	spectral	1		0	1.69s
✅ GHERKIN	gherkin-lint	3		0	1.08s
✅ REPOSITORY	git_diff	yes		no	0.02s
✅ REPOSITORY	secretlint	yes		no	0.75s
✅ YAML	yamllint	1		0	0.48s

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[XunliYang]

LGTM

[Wojiaozhenghao]

approve