Add support for enum fields

[ohadravid]

This PR makes it possible to have fields which are enums containing other WMI objects, like so:

#[derive(Deserialize, Debug)]
struct Win32_Process {}

#[derive(Deserialize, Debug)]
struct Win32_Thread {}

#[derive(Deserialize, Debug)]
enum Instance {
    #[serde(rename = "Win32_Process")]
    Process(Win32_Process),
    #[serde(rename = "Win32_Thread")]
    Thread(Win32_Thread),
}

#[derive(Deserialize, Debug)]
struct __InstanceCreationEvent {
    TargetInstance: Instance,
}

let mut filters_process = HashMap::new();

filters_process.insert(
    "TargetInstance".to_owned(),
    FilterValue::is_a::<Win32_Process>().unwrap(),
);

let mut instances_iter = wmi_con
    .filtered_notification::<__InstanceCreationEvent>(
        &filters_process,
        Some(Duration::from_secs(1)),
    )
    .unwrap();