Support response header filter for GRPCRoute

[ciarams87]

Proposed changes

Problem: As a user, I want to be able to add ResponseHeaderModifier filter for my GRPCRoutes.

Solution: Add support for ResponseHeaderModifier filter for my GRPCRoutes. The implementation reuses validation and logic from the HTTPRoute implementation. This PR also removes some redundant empty structs in the servers_test.go

Testing: The functionality is identical to HTTPRoute so not much additional testing is required. I tested locally using echo server and grpcurl. I didn't modify the application to configure headers on the upstream side, but we can see the response headers set in the GRPCRoute spec have been returned:

Response headers received:
content-type: application/grpc
date: Thu, 16 May 2024 10:06:44 GMT
server: nginx/1.25.5
x-header-add: this-is-the-appended-value
x-header-set: overwritten-value

Closes #1935

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.

Added support for Response Header Modifier filter for GRPCRoute

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.92%. Comparing base (869ac38) to head (f0e75dc).
Report is 1 commits behind head on main.

❗ Current head f0e75dc differs from pull request most recent head 556c8c7. Consider uploading reports for the commit 556c8c7 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1983      +/-   ##
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+ Coverage   86.90%   86.92%   +0.01%     
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  Files          88       88              
  Lines        6056     6064       +8     
  Branches       50       50              
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+ Hits         5263     5271       +8     
  Misses        741      741              
  Partials       52       52              

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[ADubhlaoich]

LGTM docs-wise!

[bjee19]

👍

[kate-osborn]

🚀