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/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@410904 f38db490-d61c-443f-a65b-d21fe96a405b