res_pjsip: Add "pjsip show version" CLI command.

When debugging things it can be useful to know absolutely what
version of pjproject res_pjsip is running against. This change
adds a "pjsip show version" CLI command which can be used to
query for this.

ASTERISK-24685 #close
Reported by: Joshua Colp

Review: https://reviewboard.asterisk.org/r/4424/
........

Merged revisions 431824 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@431825 65c4cc65-6c06-0410-ace0-fbb531ad65f3