app_confbridge: Add function to retrieve channels.

Adds the CONFBRIDGE_CHANNELS function which can be used
to retrieve a comma-separated list of channels, filtered
by a particular type of participant category. This output
can then be used with functions like UNSHIFT, SHIFT, POP,
etc.

ASTERISK-30036 #close

Change-Id: I1950aff932437476dc1abab6f47fb4ac90520b83