Support dynamic Host header rewrite with sourceHeader in ListenerRuleConfiguration CRD

[yaswanthkumar1995]

Issue

#4693

Description

Adds support for specifying host-header-rewrite transforms directly through the ListenerRuleConfiguration CRD, enabling multi-tenant SaaS patterns where the Host header needs to be dynamically rewritten using the value of another incoming request header (e.g., X-School-Domain).

This implements Option A from the issue — a new transforms field in the ListenerRuleConfiguration CRD with a sourceHeader reference:

apiVersion: gateway.k8s.aws/v1beta1
kind: ListenerRuleConfiguration
metadata:
  name: tenant-host-rewrite
spec:
  transforms:
    - type: host-header-rewrite
      hostHeaderRewriteConfig:
        sourceHeader: X-School-Domain
        rewrites:
          - regex: ".*"
            replace: "$0"

What changed:

File	Change
apis/gateway/v1beta1/listenerruleconfig_types.go	Add Transforms field to ListenerRuleConfigurationSpec with host-header-rewrite type, sourceHeader, and kubebuilder validation markers
pkg/model/elbv2/listener_rule.go	Add SourceHeader field to RewriteConfigObject internal model
pkg/gateway/routeutils/route_rule_transform.go	Add BuildListenerRuleConfigTransforms() to convert CRD transforms to internal model
pkg/gateway/model/model_build_listener.go	Add mergeTransforms() — merges route-level and CRD-level transforms, CRD takes precedence for same-type overrides

Design decisions:

• sourceHeader is a forward-looking field. AWS ALB does not yet natively support cross-header references (confirmed by AWS Premium Support). The field is wired through the CRD and internal model so that when AWS adds SDK support, only the SDK builder (buildSDKHostHeaderRewriteConfig) needs updating.
• CRD transforms override route-generated transforms of the same type (e.g., a CRD host-header-rewrite replaces one generated from HTTPRoute hostname matching).
• Scoped to host-header-rewrite only — no url-rewrite in the CRD since it wasn't requested.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: yaswanthkumar1995 / name: Yaswanth Kumar (661507d)

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