feat: implement a Keptn metrics provider

Add a Keptn metrics provider for two resources:
* KeptnMetric: Verify the value of a single metric.
* Analysis (via AnalysisDefinition): Run a Keptn analysis over an
  interval validating SLOs.

Signed-off-by: Florian Bacher <florian.bacher@dynatrace.com>

cmd/flagger/main.go

@@ -253,6 +253,7 @@ func main() {
 		fromEnv("EVENT_WEBHOOK_URL", eventWebhook),
 		clusterName,
 		noCrossNamespaceRefs,
+		cfg,
 	)
 
 	// leader election context