samples: net: http_server: add extra config for enable DHCPv4

Add `overlay-dhcpv4.conf` for using this sample with DHCPv4.

When the DHCPv4 client is enabled, we must have a large number
for `CONFIG_ZVFS_EVENTFD_MAX`.
This sample may be frequently used with the DHCPv4 client,
so we'll add the extra config file.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>