Update an existing webhook endpoint.

Allows partial updates - only provided fields will be modified.
The webhook must belong to the authenticated user (or account for privileged access).

An account-wide webhook is updated through this same endpoint, and needs a privileged key:
without one its id reads as not found, the same answer as another user's webhook.

scope cannot be changed after creation, and is ignored here: a webhook's scope decides
which Svix application its endpoint lives in, so converting one means deleting and
recreating it.