dhcp: T6948: Keep DHCP server leases in sync with hostd records

Keep DHCP server leases in sync with vyos-hostd records
via helper script invoked with ExecStartPost directive
in kea-dhcp4-server.service.

The helper script updates VyOS hostd records from
DHCP server leases. This ensures that hostd records
with the DHCP server leases are kept in sync with
VyOS hostd records right after DHCP server is started.

This is similar to the capability exposed via kea hook
libdhcp_run_script.so which is invoked internally
by kea when a single lease changes state.

Since the kea hook is currently implemented for DHCPv4
only, this helper script is implemented for DHCPv4
only as well.