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>

kustomize/base/flagger/crd.yaml

@@ -1298,6 +1298,7 @@ spec:
                         - newrelic
                         - graphite
                         - dynatrace
+                        - keptn
                     address:
                       description: API address of this provider
                       type: string

kustomize/base/flagger/rbac.yaml

@@ -241,6 +241,19 @@ rules:
       - update
       - patch
       - delete
+  - apiGroups:
+      - metrics.keptn.sh
+    resources:
+      - keptnmetrics
+      - analyses
+    verbs:
+      - get
+      - list
+      - watch
+      - create
+      - update
+      - patch
+      - delete
   - nonResourceURLs:
       - /version
     verbs: