Choose a time to book at a clinic

For a public_provider_booking action: keeps one time from list_public_booking_slots (or a soonest search row's earliest) and returns continue_url, Xona's page where the patient enters their own details and presses Book. Xona then books that time in the clinic's own scheduler. It takes the action and the time (service_id, slot_id, starts_at) only, reads that time again and keeps it only if it is still open; the page collects the patient's details.