Add system_extensions fact

[nixtar]

I recently had to transition from one AV product to another.

The AV product I was moving away from (Symantec) has no silent uninstall ability while the system extension is in an activated state.

This fact provides munki with an array of hashtables for each system extension. An example structure for system_extensions is as follows:

{'system_extensions': [
    {'teamID': 'UBF8T346G9', 'bundleID': 'com.microsoft.wdav.netext', 'version': '101.56.62/101.56.62', 'name': 'Microsoft Defender Network Extension', 'state': 'terminated waiting to uninstall on reboot'},
    {'teamID': 'UBF8T346G9', 'bundleID': 'com.microsoft.wdav.epsext', 'version': '101.56.62/101.56.62', 'name': 'Microsoft Defender Endpoint Security Extension', 'state': 'terminated waiting to uninstall on reboot'},
    {'teamID': 'Y2CCP3S9W7', 'bundleID': 'com.broadcom.mes.systemextension', 'version': '9.1.100/9.1.100', 'name': 'Symantec System Extension', 'state': 'activated waiting for user'}
]}