res_pjsip_endpoint_identifier_ip: Endpoint identifier request URI

res_pjsip_endpoint_identifier_ip: Endpoint identifier request URI

Add ability to match against PJSIP request URI.

UserNote: this new feature let users match endpoints based on the
indound SIP requests' URI. To do so, add 'request_uri' to the
endpoint's 'identify_by' option. The 'match_request_uri' option of
the identify can be an exact match for the entire request uri, or a
regular expression (between slashes). It's quite similar to the
header identifer.

Fixes: #599
(cherry picked from commit 136bd5c892b5c44ca5f4f6908e957404a840d60b)