add support for emitting webhooks

[baywet]

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.