Perform periodic health checks of mountpoint by agents (configurable)

[achimnol]

depends on #1845

• Add a local timer task to the agent which checks the availability of vfolder mounts using mountpoint command.
  • This can be activated on Linux only; if we could find the same mechanism on macOS, let's add it.
  • When the check fails, publish MonitoringAlarmEvent (refs Add a manager endpoint to publish monitoring alarm events #1845).
• Add an agent local configuration to enable/disable mountpoint checks.
• Include mountpoint status information in the debug interface output in feat: UNIX socket server to report agent status #2038.