DHCP custom option (#8582)

* works

* fixup! works

* back to callbacks

* names

* daisy chain

* seconds

* less inline

* fix dns setter

* might as well keep using initlist

/to d-a-v it has automatic storage, here it's the same stack based one
(just one less line for us)

* shift blame

* naming

* fix impl

* revert to ip4 dns

* merge fix

* restyle

* masking done wrong

libraries/ESP8266WiFi/examples/CustomOffer/CustomOffer.ino

@@ -0,0 +1,20 @@
+#include <Arduino.h>
+#include <ESP8266WiFi.h>
+
+void setup() {
+  auto& server = WiFi.softAPDhcpServer();
+  server.onSendOptions([](const DhcpServer& server, auto& options) {
+    // VENDOR is... vendor specific
+    options.add(43, { 0xca, 0xfe, 0xca, 0xfe, 0xfe });
+
+    // Captive Portal URI
+    const IPAddress gateway = netif_ip4_addr(server.getNetif());
+    const String captive = F("http://") + gateway.toString();
+    options.add(114, captive.c_str(), captive.length());
+  });
+  WiFi.softAP("TEST", "testtesttest");
+}
+
+void loop() {
+  delay(100);
+}