resolve: extend systemd-resolve so that it can push per-interface DNS configuration...

resolve: extend systemd-resolve so that it can push per-interface DNS configuration into systemd-resolved (#7576)

This is useful to debug things, but also to hook up external post-up
scripts with resolved.

Eventually this code might be useful to implement a
resolvconf(8)-compatible interface for compatibility purposes. Since the
semantics don't map entirely cleanly as first step we add a native
interface for pushing DNS configuration into resolved, that exposes the
correct semantics, before adding any compatibility interface.

See: #7202