res_stasis: Extend bridge type to be a comma separated list of bridge attributes.

This change turns the bridge type field into a comma separated list of attributes.
These attributes include: mixing, holding, dtmf_events, and proxy_media. By setting
the various attributes a user can control the type of bridge created with the
behavior they need for their application.

(closes issue ASTERISK-23437)
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/3359/
........

Merged revisions 410904 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@410905 65c4cc65-6c06-0410-ace0-fbb531ad65f3