FS-9287 Add channel variable to make spandsp_start_tone_detect easier to use from dialplan / embedded scripts.
The following variables are added:

execute_on_spandsp_tone_detect_<TONE NAME> : executes APP when tone is detected.  For example, if you have a tone
    named "SIT" defined in spandsp.conf.xml, the variable would be "execute_on_spandsp_tone_detect_SIT"

api_on_spandsp_tone_detect_<TONE NAME> : similar to execute_on except that it executes an API.

spandsp_tone_detect_stop_on_tone : default is false.  If true, detector will stop once the first matching tone is detected.