Add Traffic Mirroring for Istio Service Mesh

Traffic mirroring is a pre-stage for canary deployments.  When mirroring
is enabled, at the beginning of a canary deployment traffic is mirrored
to the canary instead of shifted for one canary period.  The service
mesh should mirror by copying the request and sending one copy to the
primary and one copy to the canary; only the response from the primary
is sent to the user.  The response from the canary is only used for
collecting metrics.

Once the mirror period is over, the canary proceeds as usual, shifting
traffic from primary to canary until complete.

Added TestScheduler_Mirroring unit test.

artifacts/flagger/crd.yaml

@@ -41,6 +41,10 @@ spec:
       type: string
       JSONPath: .spec.canaryAnalysis.interval
       priority: 1
+    - name: Mirror
+      type: boolean
+      JSONPath: .spec.canaryAnalysis.mirror
+      priority: 1
     - name: StepWeight
       type: string
       JSONPath: .spec.canaryAnalysis.stepWeight
@@ -183,6 +187,9 @@ spec:
                 stepWeight:
                   description: Canary incremental traffic percentage step
                   type: number
+                mirror:
+                  description: Mirror traffic to canary before shifting
+                  type: boolean
                 match:
                   description: A/B testing match conditions
                   anyOf: