Add support to digest authentication

[etr]

The definitive support to digest authentication is necessary to completely map libmicrohttpd functionalities.

[etr]

Digest authentication is definitively supported. It is now possible to obtain the username from the request through the method getDigestedUser.

It is also possible to check the authentication through an utility method of HttpRequest class.

It is now possible to build a HttpResponse with response type DIGEST_AUTH_FAIL that automatically sets useful headers.