CORS - Followup on suggested improvements

**What would you like to be added**:
Followup on suggested improvements for CORS:
- https://github.com/kubernetes-sigs/gateway-api/pull/3739/files#r2667680110 - Fix header ordering, do not introduce the IgnoreWhiteSpace
- https://github.com/kubernetes-sigs/gateway-api/pull/3739/files#r2731132648 - Introduce HeaderAsList for the ordering validation
- https://github.com/kubernetes-sigs/gateway-api/pull/3739/files#r2690157156 - Followup on allowCredentials and wildcard usage for methods (this is a proxy behavior)

**Why this is needed**:
Move with CORS promotion during RC cut, and followup with implementations on conformance improvements

/cc @snorwin @DamianSawicki @zhaohuabing

Start with the referenced PR #3739 review discussions and the repository's current CORS conformance implementation. Trace the tests and code covering header ordering, HeaderAsList, and allowCredentials with wildcard methods; done means the follow-up behavior is implemented and the relevant conformance checks pass.

Written by the indexing model from the issue text.