res_pjsip_session: Access SIPDOMAIN via Dialplan.

This feature was available in the SIP channel driver chan_sip. For example,
Asterisk is the outbound proxy and has to handle all SIP-URIs, even domains not
local to Asterisk. In that case, SIPDOMAIN is used in the Dialplan, to detect
and dial remote SIP-URIs. This change here sets the SIP destination domain of
an inbound call (SIPDOMAIN) in the SIP channel driver res_pjsip as well.

ASTERISK-26670 #close

Change-Id: I27c880dc404a3c1c6792e1ba3545475339577243