(#19162) Manage pending reboots

This commit adds a `manages_reboot_pending` feature, which indicates that the
provider is able to detect when a reboot is pending/required, and if so,
reboot the system.  This is expressed as:

    reboot { 'now':
      when => pending
    }

The windows provider supports the feature, and does so by querying the
registry.

Tests are needed.