GUACAMOLE-203: Add support for configuring the server-alive-interval SSH parameter.

guacamole-ext/src/main/resources/org/apache/guacamole/protocols/ssh.json

@@ -69,6 +69,10 @@
                 {
                     "name"  : "command",
                     "type"  : "TEXT"
+                },
+                {
+                    "name"  : "server-alive-interval",
+                    "type"  : "NUMERIC"
                 }
             ]
         },
@@ -137,4 +141,4 @@
         }
 
     ]
-}
+}