res_pjsip: add 'set_var' support on endpoints

Added a new 'set_var' option for ast_sip_endpoint(s).  For each variable
specified that variable gets set upon creation of a pjsip channel involving
the endpoint.

(closes issue ASTERISK-22868)
Reported by: Joshua Colp
Review: https://reviewboard.asterisk.org/r/3095/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@404663 f38db490-d61c-443f-a65b-d21fe96a405b