Unable to set multiple set-cookie headers

[dnbn]

What steps did you take and what happened:
RFC actually allows the server to send multiples "set-cookie" header if they don't have the same name. Unfortunately contour overwrite the header and only keep the last one.
From this code, it seems there are cases where envoy can already make an append instead of a set. See here.

responseHeadersPolicy:
  set:
    - name: set-Cookie
      value: Cookie1=cookie1
    - name: set-Cookie
      value: Cookie2=cookie2

What did you expect to happen:
When multiple set-cookie headers are set in the same httpproxy, all should be kept and appended to the header list.

Anything else you would like to add:
Another approach could be to add the ability of "append header" instead of "set" header. This has already been suggested here

Environment:

• Contour version: 1.26.0

Thanks in advance

[github-actions]

Hey @dnbn! Thanks for opening your first issue. We appreciate your contribution and welcome you to our community! We are glad to have you here and to have your input on Contour. You can also join us on our mailing list and in our channel in the Kubernetes Slack Workspace

[github-actions]

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[dnbn]

Hello, please keep this issue as fresh. I'm still interested.

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[github-actions]

The Contour project currently lacks enough contributors to adequately respond to all Issues.

This bot triages Issues according to the following rules:

• After 60d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, the Issue is closed

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• Mark this Issue as fresh by commenting
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