CEL Validation: HTTPRoute Case-Insensitive Header Name Uniqueness

[robscott]

What happened:
Unfortunately CEL is not currently capable of validating the case-insensitive uniqueness of header names in header modifier filters. This is the only standard channel validation we are still reliant on the validating webhook for. @gauravkghildiyal already developed a CEL rule for this:

// +kubebuilder:validation:XValidation:message="Must not match the same header (case-insensitive) multiple times in the same rule",rule="self.all(h1, self.exists_one(h2, h1.name.lowerAscii() == h2.name.lowerAscii()))"

Unfortunately there is a problem with CEL cost estimation that prevents this kubernetes/kubernetes#119749, @jpbetz is working on a fix for this.

Anything else we need to know?:
Given the Gateway API versioning policy to support the 5 latest minor Kubernetes versions, even when this is fixed upstream, it's going to be a long time before we can adopt this.

/lifecycle frozen

[gauravkghildiyal]

/cc