feat(sysconfig): Add DNS from interface config to resolv.conf (#5401)

Jira: https://issues.redhat.com/browse/RHEL-17961

sysconfig renderer currently only uses global dns and search domain
configuration in order to populate /etc/resolv.conf. This means it ignores
interface specific dns configuration completely. This means, when global dns
information is absent and only interface specific dns configuration is present,
/etc/resolv.conf will not have complete dns information. Fix this so that
per interface dns information is also taken into account along with global dns
configuration in order to populate /etc/resolv.conf.

Upstream PR https://github.com/canonical/cloud-init/pull/5401

Fixes: GH-5400

One conflict has to be resolved: load_text_file() is not present in c9s. Instead it has load_file(). Hence, the last hunk does not match exactly.