Allow snooping by existing channel name or unique ID

[chrsmj]

Request to add a new '-s' command line option to change the meaning of <dialstring> to instead of originating a call, be an existing channel name or unique ID already active in Asterisk.

For example, by channel name:

$ bin/ari-transcriber -s 'PJSIP/7001-00000001'

...would sic a dog on that channel and start gewgle translating it.

Similarly, by unique ID:

$ bin/ari-transcriber -s '1716337601.107'

Patch to follow.

[chrsmj]

Big thanks to Voisonics for sponsoring this patch.

[chrsmj]

FYI the patch also adds a little more logging.