ari: Add Snoop operation for spying/whispering on channels.
The Snoop operation can be invoked on a channel to spy or
whisper on it. It returns a channel that any channel operations
can then be invoked on (such as record to do monitoring).
(closes issue ASTERISK-22780)
Reported by: Matt Jordan
Review: https://reviewboard.asterisk.org/r/3003/
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Merged revisions 403117 from http://svn.asterisk.org/svn/asterisk/branches/12
git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403118
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