Add CORS policy support

[stefanprodan]

This PR allows cross-origin resource sharing policy to be specified in the Canary service spec.

apiVersion: flagger.app/v1alpha3
kind: Canary
spec:
  service:
    port: 9898
    corsPolicy:
      allowOrigin:
        - example.com
      allowMethods:
        - GET
      allowCredentials: false
      allowHeaders:
        - x-some-header
      maxAge: 24h

Fix: #58

[codecov-io]

Codecov Report

Merging #83 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #83      +/-   ##
=========================================
+ Coverage   54.74%   54.8%   +0.05%     
=========================================
  Files          13      13              
  Lines        1538    1540       +2     
=========================================
+ Hits          842     844       +2     
  Misses        556     556              
  Partials      140     140

Impacted Files	Coverage Δ
pkg/router/istio.go	81.08% <100%> (+0.25%)	⬆️

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