add support for emitting webhooks

Microsoft Graph has types that are only used when the service calls to the client's APIs: change notifications.
In the Microsoft Graph pipeline, those types are being trimmed by kiota because they are not used anywhere (any inbound endpoint).
We should:

1. come up with an annotation that means "used for a webhook"
2. add this annotation to the metadata via XOD
3. leverage that annotation here so a webhook definition can be added to the description
4. have kiota project the types from webhooks components, and not trim them.

まず Microsoft Graph pipeline、XOD metadata flow、および inbound endpoints で使用されない型を Kiota が trimming する処理を追跡します。生成された description で webhook annotations をどのように表現し、利用するかを定義します。Webhook definitions が必要な型を保持し、Kiota がそれらを trimming せずに投影できれば完了です。