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>

Author: soburi

samples/net/sockets/http_server/README.rst

@@ -37,6 +37,9 @@ There are configuration files for various setups in the
     * - :zephyr_file:`overlay-tls.conf <samples/net/sockets/http_server/overlay-tls.conf>`
       - This overlay config can be added to build the HTTPS variant.
 
+    * - :zephyr_file:`overlay-dhcpv4.conf <samples/net/sockets/http_server/overlay-dhcpv4.conf>`
+      - This overlay enables DHCPv4 client feature.
+
 To build and run the HTTP server application:
 
 .. code-block:: bash

samples/net/sockets/http_server/overlay-dhcpv4.conf

@@ -0,0 +1,2 @@
+CONFIG_NET_DHCPV4=y
+CONFIG_ZVFS_EVENTFD_MAX=2