res_pjsip_session: Add configurable behavior for redirects.

The action taken when a redirect occurs is now configurable on a
per-endpoint basis. The redirect can either be treated as a redirect
to a local extension, to a URI that is dialed through the Asterisk
core, or to a URI that is dialed within PJSIP itself.

(closes issue ASTERISK-21710)
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/2963/
........

Merged revisions 403207 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403208 65c4cc65-6c06-0410-ace0-fbb531ad65f3