Support attaching ClientTrafficPolicy to HTTPRoute for fine-grained mTLS control

*Description*:
Hi,

I'm currently working with mTLS configuration in Envoy Gateway using ClientTrafficPolicy, and I've run into a limitation that makes fine-grained control difficult in real-world scenarios.

Current behavior

Based on the CRD definition, ClientTrafficPolicy:

Can only target Gateway resources (via targetRefs / targetSelectors)
Must be in the same namespace as the targeted Gateway
Can only be attached at the listener level, not to individual HTTPRoutes

This design makes it difficult to apply mTLS selectively to specific endpoints.

In particular:

I cannot attach a ClientTrafficPolicy directly to an HTTPRoute

As a result, I am forced to:
Create dedicated listeners in the Gateway
Route specific endpoints through those listeners