feat: support custom backendRef via extension server

[Xunzhuo]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #4762

Release Notes: Yes

This PR added support for custom backend, it is used widely in vendor specific Backend. Also it is used in Envoy AI Gateway:

apiVersion: inference.networking.x-k8s.io/v1alpha2
kind: InferencePool
metadata:
  name: vllm-llama3-8b-instruct
spec:
  targetPortNumber: 8000
  selector:
    app: vllm-llama3-8b-instruct
  extensionRef:
    name: vllm-llama3-8b-instruct-epp
---
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: backend
spec:
  parentRefs:
    - name: eg
  hostnames:
    - "www.example.com"
  rules:
    - backendRefs:
        - group: inference.networking.x-k8s.io/v1alpha2
          kind: InferencePool
          name: vllm-llama3-8b-instruct
      matches:
        - path:
            type: PathPrefix
            value: /

[codecov]

Codecov Report

Attention: Patch coverage is 74.44934% with 58 lines in your changes missing coverage. Please review.

Project coverage is 70.85%. Comparing base (71ce56f) to head (3ad654d).

Files with missing lines	Patch %	Lines
internal/provider/kubernetes/controller.go	53.19%	20 Missing and 2 partials ⚠️
internal/provider/kubernetes/routes.go	65.38%	6 Missing and 3 partials ⚠️
internal/xds/translator/extension.go	66.66%	6 Missing and 3 partials ⚠️
internal/provider/kubernetes/filters.go	69.56%	5 Missing and 2 partials ⚠️
internal/extension/registry/extension_manager.go	0.00%	4 Missing ⚠️
internal/extension/registry/xds_hook.go	83.33%	2 Missing and 1 partial ⚠️
internal/gatewayapi/runner/runner.go	0.00%	3 Missing ⚠️
internal/gatewayapi/route.go	98.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6342      +/-   ##
==========================================
+ Coverage   70.76%   70.85%   +0.09%     
==========================================
  Files         220      220              
  Lines       37382    37575     +193     
==========================================
+ Hits        26452    26623     +171     
- Misses       9382     9397      +15     
- Partials     1548     1555       +7     

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[Xunzhuo]

/retest

[Xunzhuo]

/retest

[Xunzhuo]

/retest

[Xunzhuo]

/retest

[arkodg]

LGTM thanks !

[guydc]

LGTM, thanks!

In general, we need to prioritize investment in E2Es for the extensions server. A lot of users and projects are increasingly relying on this feature which is currently not E2E tested.

[Xunzhuo]

/retest

[Xunzhuo]

/retest