nixos: Add option networking.networkmanager.dynamicHosts

This allows non-privileged users to configure local DNS
entries by editing hosts files read by NetworkManager's dnsmasq
instance.

Cherry-picked from e6c3d5a507909c4e0c0a5013040684cce89c35ce and
5a566004a2b12c3d91bf0acdb704f1b40770c28f.