DNS: Create a system-level DNS resolver

Prior to this patch, the DNS core present in master had no default system-level
resolver implementation. Therefore, it was not possible for the DNS core to
perform resolutions unless the libunbound library was installed and the
res_resolver_unbound module was loaded.

This patch introduces a system-level DNS resolver implementation that will
register itself with the lowest consideration priority available (to ensure
that it is to be used only as a last resort). The resolver relies on low-level
DNS search functions to perform a rudimentary DNS search based on a provided
query and then supplies the search results to the DNS core.

ASTERISK-25146 #close
Reported By: Joshua Colp

Change-Id: I3b36ea17b889a98df4f8d80d50bb7ee175afa077