Add support for HTTP request header manipulation rules

[stefanprodan]

This PRs adds support for appending headers to HTTP requests.

apiVersion: flagger.app/v1alpha3
kind: Canary
spec:
  service:
    port: 9898
    headers:
      request:
        add:
          x-envoy-upstream-rq-timeout-ms: "15000"
          x-envoy-max-retries: "10"
          x-envoy-retry-on: "gateway-error,connect-failure,refused-stream"

Fix: #81

[codecov-io]

Codecov Report

Merging #82 into master will increase coverage by 0.17%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
+ Coverage   54.56%   54.74%   +0.17%     
==========================================
  Files          13       13              
  Lines        1532     1538       +6     
==========================================
+ Hits          836      842       +6     
  Misses        556      556              
  Partials      140      140

Impacted Files	Coverage Δ
pkg/router/istio.go	80.82% <90%> (+0.82%)	⬆️

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