Add ICMP to the list of supported protocols (#3993)

Author: fanymanea

specification/network/resource-manager/Microsoft.Network/stable/2018-08-01/azureFirewall.json

@@ -741,7 +741,8 @@
          "enum":[  
             "TCP",
             "UDP",
-            "Any"
+            "Any",
+            "ICMP"
          ],
          "x-ms-enum":{  
             "name":"AzureFirewallNetworkRuleProtocol",