res_stasis_playback: Add 'number', 'digits', and 'characters' URI scheme implementations.

This change adds new URI scheme implementations for playing numbers, digits,
and characters. This is done as part of the normal playback mechanism and can
be used with queueing to create a combined sentence.

Review: https://reviewboard.asterisk.org/r/3028/
........

Merged revisions 403209 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403210 65c4cc65-6c06-0410-ace0-fbb531ad65f3