Creates a new session (Workshop slot) under a WorkshopDefinition.
Required fields: start_date and end_date. The session inherits
the WorkshopDefinition's name/description. Streaming, chat, Zoom and
related integrations are not configurable via this API.

Precondition: the event must have multiple sessions enabled; otherwise the
request fails with 422 "You cannot create a workshop session because multiple sessions are disabled for this event.". A workshop's first session is created
through createWorkshop (via workshops_attributes).