Implement webhooks model

[Azquelt]

• Note that I haven't made a WebhookIO class.
  • For the model, the new webhooks property just holds a Map<String, PathItem>
  • For the annotation, a @Webhook annotation represents one entry in that map. Since the value is a PathItem, I added it to PathItemIO. I could make a new IO class for it instead, though there's no model object to read and write to JSON.
• I haven't added any tests here because I think the TCK tests are sufficient.

Implements eclipse/microprofile-open-api#612

~In order to pass the TCK locally, I also have to merge in #1818 because that implements eclipse/microprofile-open-api#604 which has already been merged.~

[Azquelt]

Rebased on main-4.0

[Azquelt]

The annotation has been removed from the upstream PR. Removed the implementation here to match.