Returns the account-level custom field definitions applicable to every event in the
authenticated user's account. This endpoint is account-scoped, not event-scoped:
it takes no uuid path parameter. The id of each definition (cast to string) is the
key expected inside custom_fields_data on createEvent / updateEvent.

Only active fields are returned (archived: false, draft: false), ordered by
order ASC.

Caching: Responses are cached for 30 minutes.