Werk #13857: Windows agent uses perflib to obtain wmi_cpuload data

Windows agent uses now the Perflib API for service wmi_cpuload as a more reliable and stable source of information compared to the previously used WMI API.

If, for some reasons, the new functionality causes problems, it is possible to return to the WMI API, using the appropriate key in the user config. To do it, set in section global the key cpuload_method to 'use_wmi'.

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