Implementing GEP-1767 CORS Filter for HTTPRoute

[robscott]

What type of PR is this?
/kind feature

What this PR does / why we need it:
This is an implementation of GEP #1767 as written by @lianglli. @mkosieradzki started the work on this PR and I finished it off with some codegen and validation.

Does this PR introduce a user-facing change?:

A new CORS filter has been added to HTTPRoute

[k8s-ci-robot]

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[youngnick]

LGTM once tests pass. (I'll wait to give the command in case there are more code changes required)

[kflynn]

Looks good from here once tests pass, too.

[robscott]

This has turned into quite the rabbit hole to get codegen working, CRDs properly sized, and presubmits passing. I may end up splitting some of these changes out into separate PRs if/when I get this working.

/hold

[robscott]

I think this is in a good state now + tests are passing. I managed to avoid the scale of changes I'd originally expected here, so will not split this up.

/hold cancel

[kflynn]

Quite the road getting this last bit in!

/lgtm

[AdamS3x]

Can I ask when this is likely going to be released? I am working on an implementation of Gateway API currently with Istio and require CORS support in HTTPRoute

[youngnick]

This will be included in Gateway API v1.3, due out March 24th.