netsock2: Add ast_sockaddr_resolve_first_af to netsock2 public API

This function originally was used in chan_sip to enable some simplifying
assumptions and eventually was copy and pasted into res_pjsip_logger and
res_hep.  Since it's replicated in three places, it's probably best to
move it into the public netsock2 API for these modules to use.

Change-Id: Id52e23be885601c51d70259f62de1a5e59d38d04