res_stasis_recording: Add a "target_uri" field to recording events.

This change adds a target_uri field to the live recording object. It
contains the URI of what is being recorded.

(closes issue ASTERISK-23258)
Reported by: Ben Merrills

Review: https://reviewboard.asterisk.org/r/3299/
........

Merged revisions 410025 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@410027 65c4cc65-6c06-0410-ace0-fbb531ad65f3