Add FreezerState to UnitStatus

[mbssrc]

Description

Adding indicator for frozen (pause) units.

• add field 'FreezerState' to UnitStatus as returned by givc agent
• returns 'error' if it could not be retrieved

Checklist

• Summary of the proposed changes in the PR description
• More detailed description in the commit message(s)
• Commits are squashed into relevant entities - avoid a lot of minimal dev time commits in the PR
• Contribution guidelines followed
• Test procedure added to nixos/tests
• Author has run nix flake check --accept-flake-config and it passes
• All automatic Github Action checks pass - see actions
• Author has added reviewers and removed PR draft status

Testing

[ghaf@gui-vm:~]$ grpcurl -plaintext -d '{"UnitName":"[email protected]"}' chromium-vm:9000 systemd.UnitControlService.FreezeUnit
{
  "CmdStatus": "Command successful"
}

[ghaf@gui-vm:~]$ grpcurl -plaintext -d '{"UnitName":"[email protected]"}' chromium-vm:9000 systemd.UnitControlService.GetUnitStatus
{
  "CmdStatus": "Command successful",
  "UnitStatus": {
    "Name": "[email protected]",
    "Description": "Application service for chromium",
    "LoadState": "loaded",
    "ActiveState": "active",
    "SubState": "running",
    "Path": "/org/freedesktop/systemd1/unit/chromium_400_2eservice",
    "FreezerState": "frozen"
  }
}

[ghaf@gui-vm:~]$ grpcurl -plaintext -d '{"UnitName":"[email protected]"}' chromium-vm:9000 systemd.UnitControlService.UnfreezeUnit
{
  "CmdStatus": "Command successful"
}

[ghaf@gui-vm:~]$ grpcurl -plaintext -d '{"UnitName":"[email protected]"}' chromium-vm:9000 systemd.UnitControlService.GetUnitStatus
{
  "CmdStatus": "Command successful",
  "UnitStatus": {
    "Name": "[email protected]",
    "Description": "Application service for chromium",
    "LoadState": "loaded",
    "ActiveState": "active",
    "SubState": "running",
    "Path": "/org/freedesktop/systemd1/unit/chromium_400_2eservice",
    "FreezerState": "running"
  }
}