Backport of NET-5531 Translate response header modifier(s) from HTTPRoute onto Consul config entry into release/1.2.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #2904 to be assessed for backporting due to the inclusion of the label backport/1.2.x.

🚨

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The person who merged in the original PR is:
@nathancoleman
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: POST https://api.github.com/repos/hashicorp/consul-k8s/merges: 409 Merge conflict []

The below text is copied from the body of the original PR.

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Note
This PR depends on changes in hashicorp/consul#18646

Changes proposed in this PR:
Translate the responseHeaderModifier that already exists on the K8s HTTPRoute onto the http-route config entry in Consul, which accepts response header modifiers as of hashicorp/consul#18646.

This PR also addresses the fact that we write an empty header modifier to Consul today for request header modifications when the K8s HTTPRoute does not specify any header modifications. Instead, we now appropriately write an empty list of header modifications to Consul when the HTTPRoute does not specify any.

How I've tested this PR:
Create a K8s HTTPRoute with various response header modifier filter(s) and verify that the filter syncs into Consul and functions correctly. See video of my own testing.

In addition, conformance tests exist here for this functionality that should pass where they previously failed. Here is the output of those tests passing.

How I expect reviewers to test this PR:
See above

Checklist:

• Tests added
• CHANGELOG entry added

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Overview of commits

• 1690cfa - c92e491 - a68d670 - 30f5c47 - 66e302b - 65d373a - 794f9c4

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[nathancoleman]

Decided not to backport