res_pjsip: Add support for dnsmgr to external_media_address.

The "external_media_address" option on transports is now
resolved using dnsmgr. This allows it to be automatically
refreshed regularly if refreshes are enabled in dnsmgr.
If the system is using a dynamic IP address a dynamic DNS
hostname can be provided to keep the IP address up to
date.

Change-Id: Ia54771720dff0105bde55d5bbb81a3ba437e05b2