Adds a host to an event session

Adds a user as a host of the event session. Atomic and idempotent: adding a user who is already a host returns 200 without duplicating. The user must belong to your organization. Hosts are readable on the session as host_user_ids and usable in message templates via the {{ event-session.hosts }}, {{ event-session.host }}, and {{ event-session.host-names }} merge tags.