Add configuration for URL probes to Apple profile

Chrome and Android both request a known URL that generates HTTP 204 No Content responses to determine if they have internet connectivity. In Apple profiles, we can use the same URL to determine whether the VPN needs to connect. Using this feature will help save battery life for lots of users.

roles/vpn/templates/mobileconfig.j2

@@ -37,6 +37,8 @@
 {% endif %}
                     <key>InterfaceTypeMatch</key>
                       <string>WiFi</string>
+                    <key>URLStringProbe</key>
+                      <string>http://www.gstatic.com/generate_204</string>
                   </dict>
                   <dict>
                     <key>Action</key>
@@ -47,6 +49,8 @@
 {% endif %}
                     <key>InterfaceTypeMatch</key>
                       <string>Cellular</string>
+                    <key>URLStringProbe</key>
+                      <string>http://www.gstatic.com/generate_204</string>
                   </dict>
                 </array>
 {% else %}