[sr-dev] [kamailio/kamailio] registrar: adding contact and UA to xavp_cfg (Issue #3753)

### Description

We are using Kamailio as a 'mid-registrar', and saving the location record (i.e calling registrar.save()) in the reply-route depending on received status code from other registrar.

We noticed the saved location record had the contact URI (incl. instance id) and UA from response message; instead we would like to use the contact and UA from the original request.

This feature change is similar to what was added for `tcpconn_id` in PR: https://github.com/kamailio/kamailio/pull/3430. i.e. we would set the request's contact URI and UA in `xavp_cfg` variable. Then `registrar.save.c` would use the contact and UA from `xavp_cfg`.

Thanks.