RTIR uses the SkipNotification argument to Create (added in 7e0e0a54) to
squelch whole classes of users at create time.  2047c321 added a
TransSquelchTo argument to RT::Ticket->Create to implement this
create-time squelching, but didn't allow explicit control of individual
addresses at the level of the CreateTicket API.

Add the same TransSquelchTo argument to RT::Interface::Email's
CreateTicket, which allows an explicit additional set of addresses to
squelch, in addition to those that SkipNotification adds.