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

sftpd/ssh_cmd.go

@@ -747,7 +747,7 @@ func (c *sshCommand) sendExitStatus(err error) {
 				targetPath = p
 			}
 		}
-		common.SSHCommandActionNotification(&c.connection.User, cmdPath, targetPath, c.command, err)
+		common.ExecuteActionNotification(&c.connection.User, "ssh_cmd", cmdPath, targetPath, c.command, common.ProtocolSSH, 0, err)
 	}
 }