Add support for ResponseHeaderModifier for HTTPRouteRule objects

[salonichf5]

Proposed changes

Problem: Users want to add, set, and remove response headers.

Solution: Use add_header NGINX directive to support ResponseHeaderModifier.

• If the action is set, we simply configure the add_header directive with the given value in the HTTPRoute spec.
• If the action is remove, we configure the add_header directive with the value set to an empty string.
• If the action is add, we create a mapping for the $http_header_name variable appending a comma at the end of any client provided headers (if present) and prepend this to the given value in the HTTPRoute spec.

Testing: Describe any testing that you did.

Please focus on (optional): If you any specific areas where you would like reviewers to focus their attention or provide
specific feedback, add them here.

Closes #1397

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

Introduces support for the HTTP filter `ResponseHeaderModifier`, enabling the modification of response headers within HTTPRoutes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.83%. Comparing base (d264258) to head (2fc154f).
Report is 14 commits behind head on main.

Additional details and impacted files

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+ Coverage   86.72%   86.83%   +0.11%     
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  Files          88       88              
  Lines        5972     6025      +53     
  Branches       50       50              
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+ Hits         5179     5232      +53     
  Misses        741      741              
  Partials       52       52              

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[salonichf5]

Kevin's PR - #1494

[salonichf5]

Shouldn't we be adding the responseHeaderModifier validation here ? @kate-osborn

[kate-osborn]

Shouldn't we be adding the responseHeaderModifier validation here ? @kate-osborn

@salonichf5 it looks like the intent was to make the HTTPRequestHeaderValidator more generic so it could validate both request and response headers, and then add the response header-specific validation separately in the validateFilterResponseHeaderModifier function in httproute.go.

I like your suggestion of adding a new struct to the HTTPValidator that validates response headers. I would move the functionality of validateFilterResponseHeaderModifier to that new validator.

Edit: @salonichf5 after looking through the code again, I think there will be less code duplication if we keep the approach the same. I would just rename HTTPRequestHeaderValidator to HTTPHeaderValidator and keep everything else the same.

[sjberman]

Btw when we squash and merge, we can add @kevin85421 as a co-author to the commit so proper credit is given :)

[kevin85421]

@sjberman Thank you! You guys are so nice.

[ADubhlaoich]

LGTM docs-wise but the PR is primarily code based so I will not approve.