Add support for Istio traffic policy

[stefanprodan]

This PR allows customising the Istio load balancing, connection pool sizes and outlier detection by extending the canary service spec with traffic policy.

Example:

apiVersion: flagger.app/v1alpha3
kind: Canary
spec:
  service:
    trafficPolicy:
      loadBalancer:
        consistentHash:
          httpCookie:
            name: user
            ttl: 0s

Fix: #199
Ref: #67