app_directory: add ability to specify configuration file

Adds option to app_directory to specify a filename from which to
read configuration instead of voicemail.conf ie;

same => n,Directory(,,c(directory.conf))

This configuration should contain a list of extensions using the
voicemail.conf format, ie;

2020=2020,Dog Dog,,,,attach=no|saycid=no|envelope=no|delete=no

ASTERISK-30404

Change-Id: Id58ccb1344ad1e563fa10db12f172fbd104a9d13