While it is possible to create multiple mixmonitor instances
on a channel, it was not previously possible to mute individual
instances.

This change includes the ability to specify the MixMonitorID
when calling the manager action: MixMonitorMute.  This will
allow an individual MixMonitor instance to be muted via id.
This id can be stored as a channel variable using the 'i'
MixMonitor option.

As part of this change, if no MixMonitorID is specified in
the manager action MixMonitorMute, Asterisk will set the mute
flag on all MixMonitor spy-type audiohooks on the channel.
This is done via the new audiohook function:
ast_audiohook_set_mute_all.

ASTERISK-30464

Change-Id: Ibba8c7e750577aa1595a24b23316ef445245be98
(cherry picked from commit fa635a872ea410d656d1f912a49bae66e95f1ae9)