The key functions added are:

`bool enableForwarder(const String &domainName=emptyString, const IPAddress &dns=uint32_t)0)`

If specified, `enableForwarder` will update the `domainName` that is used to match DNS request to this AP's IP Address. A non-matching request will be forwarded to the DNS server specified by `dns`. 

Returns `true` on success.

Returns `false`, 
 * when forwarding `dns` is not set, or 
 * unable to allocate resources for managing the DNS forward function.

`void disableForwarder(const String &domainName=emptyString, bool freeResources=false)`

`disableForwarder` will stop forwarding DNS requests. If specified, updates the `domainName` that is matched for returning this AP's IP Address.
Optionally, resources used for the DNS forward function can be freed.