allow to execute actions for file operations and SSH commands synchronously

The actions to run synchronously can be configured via the `execute_sync`
configuration key.

Executing an action synchronously means that SFTPGo will not return a result
code to the client until your hook have completed its execution.

Fixes #409

sftpgo.json

@@ -4,6 +4,7 @@
     "upload_mode": 0,
     "actions": {
       "execute_on": [],
+      "execute_sync": [],
       "hook": ""
     },
     "setstat_mode": 0,