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.

首先追踪 Microsoft Graph pipeline、XOD metadata flow，以及 Kiota 对未被 inbound endpoints 使用的类型进行 trimming 的过程。定义 webhook annotations 应如何在生成的描述中表示和使用。完成标准是 webhook definitions 保留所需类型，并且 Kiota 对其进行投影，而不是将其 trimming 掉。