app_directory: Add ADSI support to Directory.

app_directory: Add ADSI support to Directory.

This adds optional ADSI support to the Directory
application, which allows callers with ADSI CPE
to navigate the Directory system significantly
faster than is possible using the audio prompts.
Callers can see the directory name (and optionally
extension) on their screenphone and confirm or
reject a match immediately rather than waiting
for it to be spelled out, enhancing usability.

Resolves: #356
(cherry picked from commit 72c6e95ed1e59bf976c23d5267c776cfb92d0b2b)