Handle duplicate kinds in listener allowed kinds

[kate-osborn]

It's possible for a user to configure the following listener:

apiVersion: gateway.networking.k8s.io/v1
kind: Gateway
metadata:
  name: gateway
spec:
  gatewayClassName: nginx
  listeners:
  - name: http
    port: 80
    protocol: HTTP
    hostname: "*.example.com"
    allowedRoutes:
      kinds:
        - kind: HTTPRoute
        - kind: HTTPRoute

which has duplicate kinds in the allowedRoutes field. NGF then writes those duplicate kinds back to the status in the supported kinds field:

Supported Kinds:
  Group:  gateway.networking.k8s.io
  Kind:   HTTPRoute
  Group:  gateway.networking.k8s.io
  Kind:   HTTPRoute

A/C:

• Remove any duplicate kinds from SupportedKinds