Node-is-quorum-critical health check: correct enum tagging

API response is "flat", as in, it is not wrapped
into an object representing those variants.

So parse them by mapping fields to top-level JSON
document keys.

src/responses.rs

@@ -593,6 +593,7 @@ pub struct DefinitionSet {
 }
 
 #[derive(Debug, Deserialize, Clone, Eq, PartialEq)]
+#[serde(untagged)]
 pub enum HealthCheckFailureDetails {
     AlarmCheck(ClusterAlarmCheckDetails),
     NodeIsQuorumCritical(QuorumCriticalityCheckDetails),