Add Webhooks model APIs

[Azquelt]

• add OpenAPI.webhooks
• add TCKs using model reader and static file
• ~add @Webhook annotation and @OpenAPIDefinition.webhooks~
• ~add TCKs using the @Webhook annotation~

For #583

[Azquelt]

Realised that @Webhook should be added to the list of annotations in the spec as well. I'll do this in another PR so that it doesn't conflict with #607

[Azquelt]

I've now updated the list of annotations in the spec.

[Azquelt]

Rebased on main