GUACAMOLE-203: Add configuration option for SSH keepalives in SFTP connections.

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

@@ -318,6 +318,10 @@
                 {
                     "name"  : "sftp-directory",
                     "type"  : "TEXT"
+                },
+                {
+                    "name"  : "sftp-keepalive",
+                    "type"  : "NUMERIC"
                 }
             ]
         }

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

@@ -131,6 +131,10 @@
                 {
                     "name"  : "sftp-directory",
                     "type"  : "TEXT"
+                },
+                {
+                    "name"  : "sftp-keepalive",
+                    "type"  : "NUMERIC"
                 }
             ]
         },