luci-mod-network: support dhcp.ipset as section

Allow configuring ipset sections in the dhcp config:
https://openwrt.org/docs/guide-user/base-system/dhcp#ip_sets
This can also be combined with instance-specific IP sets:
https://github.com/openwrt/luci/pull/5402
However, it makes easier to assign domains to IP sets.

Signed-off-by: Vladislav Grigoryev <vg.aetera@gmail.com>