Feat cors eg

[kkk777-7]

What type of PR is this?

/kind feature

What this PR does / why we need it:
Support CORS in the Envoy Gateway emitter.

Currently, Envoy Gateway GA supports CORS via CRD/SecurityPolicy.
So, when a user selects the Envoy Gateway emitter, it should generate CRD without requiring --allow-experimental-gw-api.
when user specify --allow-experimental-gw-api while selecting EG emitter, it will generate CORS filter. #303
Also In the future, once Gateway API GA supports CORS, the common emitter should take precedence and generate a CORS filter.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

support CORS feature in Envoy Gateway Emitter.

[k8s-ci-robot]

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[Stevenjin8]

@kkk777-7

CORS just became GA in Gateway API :)

https://gateway-api.sigs.k8s.io/reference/spec/#httpcorsfilter and gateway api 1.5 is coming out soon, so I think, if anything, we should have it always emit gwapi CORS Filter regardless of --allow-experimental.

[kkk777-7]

@Stevenjin8 thanks for comments!
that makes sense, so I close this PR :)