OpenApi document: define webhooks

[tusmester]

Add webhook creation definitions to the OpenApi document if possible. These definitions describe our api for creating webhooks automatically by the 3rd party service when a custom connector is used. For details, see this article:

https://learn.microsoft.com/en-us/connectors/custom-connectors/create-webhook-trigger

❗ First check if generic OData requests for creating WebhookSubscription contents is sufficient!

If not, we may have to define a number of default/built-in webhook creator requests that create specific kinds of webhooks - e.g. a webhook for file upload, another for modification, etc.