Setup AuthN for services with no native login

[xunholy]

Details

Services such as Prometheus don't have native login oauth2 integration, this can be provided using envoy filters similar to the kiali envoy filter to enforce traffic is authenticated prior to hitting the downstream service via the virtual service resource.

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[xunholy]

Some small discussion was had here #70

[xunholy]

meshConfig:
    extensionProviders:
      - name: oauth2-proxy
        envoyExtAuthzHttp:
          service: oauth2-proxy.network.svc.cluster.local
          port: 80
          includeHeadersInCheck: ['authorization', 'cookie']
          headersToUpstreamOnAllow:
            [
              'authorization',
              'path',
              'x-auth-request-user',
              'x-auth-request-email',
              'x-auth-request-access-token',
            ]
          headersToDownstreamOnDeny: ['content-type', 'set-cookie']

      v1.9.2 implements external providers which can now easily integrate with oauth2-proxy + DEX