res_pjsip_nat: Add NAT module to session dialogs.

res_pjsip_nat: Add NAT module to session dialogs.

Due to the way pjproject internally works it was possible for the
NAT module to not be invoked on messages with-in a session dialog.
This means that the various parts of the message would not get rewritten
with the source IP address and port.

This change uses a session supplement to add the NAT module
to the dialog on the first incoming or outgoing INVITE.

(closes issue ASTERISK-22941)
Reported by: Leif Madsen
........

Merged revisions 403510 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403511 65c4cc65-6c06-0410-ace0-fbb531ad65f3