res_pjsip_path.c: Fix path when dialing using PJSIP_DIAL_CONTACTS()

When using the PJSIP_DIAL_CONTACTS() function for use in the Dial()
command, the contacts are returned in text form, so the input to
the path_outgoing_request() function is a contact value of NULL.
The issue was reported in ASTERISK-28211, but was not actually fixed
in ASTERISK-30100. This fix brings back the code that was previously
removed and adds code to search for a contact to extract the path
value from it.

(cherry picked from commit aeefedb08679c170d8bb000b4eb41093390fd13e)