pjsip: Add 'PJSIP_AOR' and 'PJSIP_CONTACT' dialplan functions.

The PJSIP_AOR dialplan function allows inspection of configured AORs including
what contacts are currently bound to them.

The PJSIP_CONTACT dialplan function allows inspection of contacts in existence.
These can include both externally added (by way of registration) or permanent
ones.

ASTERISK-24341
Reported by: xrobau

Review: https://reviewboard.asterisk.org/r/4308/
........

Merged revisions 430179 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@430180 65c4cc65-6c06-0410-ace0-fbb531ad65f3