Add support for RequestMirror filter

[sjberman]

Problem: As a user, I want to be able to mirror requests to another backend(s) using the RequestMirror filter on an HTTPRoute or GRPCRoute.

Solution: Add support for the RequestMirror filter, allowing users to mirror requests with HTTPRoutes or GRPCRoutes.

Testing: Manually verified single mirror filter, multiple mirror filter, mirror filter with other filters.

Note: the RequestMirror conformance tests seem to have a bug in 1.2.1 Gateway API, which may be fixed in 1.3. We can hopefully enable them at that point.

Closes #533

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.

Add support for request mirroring using the RequestMirror filter.

[codecov]

Codecov Report

Attention: Patch coverage is 90.09585% with 31 lines in your changes missing coverage. Please review.

Project coverage is 86.20%. Comparing base (8d5ef4d) to head (0b0bec8).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/mode/static/state/validation/validator.go	0.00%	14 Missing ⚠️
internal/mode/static/state/graph/grpcroute.go	90.29%	8 Missing and 2 partials ⚠️
internal/mode/static/state/graph/backend_refs.go	88.46%	2 Missing and 1 partial ⚠️
internal/mode/static/state/graph/httproute.go	96.38%	2 Missing and 1 partial ⚠️
...e/static/nginx/config/validation/http_validator.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3306      +/-   ##
==========================================
+ Coverage   86.16%   86.20%   +0.04%     
==========================================
  Files         113      116       +3     
  Lines       11664    11928     +264     
  Branches       62       62              
==========================================
+ Hits        10050    10283     +233     
- Misses       1553     1580      +27     
- Partials       61       65       +4     

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[ciarams87]

Note: the RequestMirror conformance tests seem to have a bug in 1.2.1 Gateway API, which may be fixed in 1.3. We can hopefully enable them at that point.

Do we have something tracking this?

Other than that, nice work! 🚀

[sjberman]

@ciarams87 I was planning on adding it as acceptance criteria to the 1.3 story when we create that

[bjee19]

lgtm! 🚀