pjsip: Add PJSIP_PARSE_URI_FROM dialplan function.

pjsip: Add PJSIP_PARSE_URI_FROM dialplan function.

Various SIP headers permit a URI to be prefaced with a `display-name`
production that can include characters (like commas and parentheses)
that are problematic for Asterisk's dialplan parser and, specifically
in the case of this patch, the PJSIP_PARSE_URI function.

This patch introduces a new function - `PJSIP_PARSE_URI_FROM` - that
behaves identically to `PJSIP_PARSE_URI` except that the first
argument is now a variable name and not a literal URI.

Fixes #756