feat: configure edgecache for postgresql

- add fact to record system resolvers
- add resolvers feature in /etc/nginx/conf.d/resolvers.conf
- add rewrite rules for postgres/yum/repodata

modules/libs/lib/facter/nameservers.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+Facter.add(:nameservers) do
+  confine kernel: 'Linux'
+  setcode do
+    nameservers = File.readlines('/etc/resolv.conf').grep(/^nameserver\s+(\S+)/) { Regexp.last_match(1) }
+    nameservers
+  end
+end