Update schedule assignees

Replaces the assignment rule for a schedule. The selected option replaces the current assignees entirely. Use 'users' to assign specific users, 'groups' to assign all members of a group, 'site_members' to assign all members of the target site(s), or 'site_members_in_group' to assign site members who are also in a group. The site_members and site_members_in_group options require the schedule to have targets (sites or assets) set.