pbx: Add variable substitution API for extensions

Currently, variable substitution involving dialplan
extensions is quite clunky since it entails obtaining
the current dialplan location, backing it up, storing
the desired variables for substitution on the channel,
performing substitution, then restoring the original
location.

In addition to being clunky, things could also go wrong
if an async goto were to occur and change the dialplan
location during a substitution.

Fundamentally, there's no reason it needs to be done this
way, so new API is added to allow for directly passing in
the dialplan location for the purposes of variable
substitution so we don't need to mess with the channel
information anymore. Existing API is not changed.

ASTERISK-29745 #close

Change-Id: I23273bf27fa0efb64a606eebf9aa8e2f41a065e4