Initialize SoftAP DhcpServer object on demand (#8546)

* Initialize SoftAP DhcpServer object on demand

Remove dependency on global ctor, and just construct the object when
someone asks us to do it. Only dependency right now is netif_git, which
is expected to be initialized by the lwip code some time before
dhcps_start happens.

Removing ip_info from begin(), since we never reference later on.
Also removing the specific check for netif id and simplify the ctors.

Update tests and recover old nonos-sdk dhcps functions that were not implemented.

* nonos helpers have a separate header

* wifi ap needs this anyway, simplify sketch includes

* missing example

* existing name :/

* trying to fix header dependency

* restyle

* not a c header

* no need to init

* move dhcp server getter to WiFi

more... arduino'ish? we ahve object as namespace, plus everything else
related to softAP is there
redundant includes, redundant mock impl (out-of-scope here to fix)

* ...move things back, still expose as WiFi method

* review fix

* include -nonos header in wifi lib though

* no more lwip include

* style

* need mock dhcpserver instance

libraries/DNSServer/examples/NAPTCaptivePortal/NAPTCaptivePortal.ino

@@ -74,7 +74,6 @@
 #include <ESP8266WiFi.h>
 #include <lwip/napt.h>
 #include <lwip/dns.h>
-#include <dhcpserver.h>
 #include <WiFiClient.h>
 #include <ESP8266WebServer.h>
 #include <DNSServer.h>