Add missing headers without overwriting in the `requestHeaderPolicy`

[lmickh]

Support adding headers that are missing without overwriting existing headers via the requestHeaderPolicy.

The requestHeaderPolicy on an httpproxy supports set which uses Envoy's OVERWRITE_IF_EXISTS_OR_ADD. If an option was included that used ADD_IF_ABSENT, we would be able to add missing headers without overwriting.

It would be nice to be able to do this via Gateway API resources as well, but that might be out of the scope of this project.

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[lmickh]

Refreshio

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[github-actions[bot]]

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

You can:

• Mark this Issue as fresh by commenting
• Close this Issue
• Offer to help out with triage

Please send feedback to the #contour channel in the Kubernetes Slack