feat(provider): implement whitelist-source-range annotation

[google-labs-jules]

This commit implements the nginx.ingress.kubernetes.io/whitelist-source-range annotation for the experimental Kubernetes Ingress provider. It extends the ingressConfig to parse the new annotation, adds an applyWhitelistSourceRange function to create and apply the ip-whitelist middleware, and includes a unit test to verify that the annotation is parsed correctly.

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PR created automatically by Jules for task 7956793084045469439 started by @stennkool

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[google-labs-jules]

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This pull request implements the nginx.ingress.kubernetes.io/whitelist-source-range annotation for the experimental Kubernetes Ingress provider. When this annotation is present on an Ingress resource, it configures a middleware that allows requests only from the specified IP source ranges.

To achieve this, I leveraged Traefik's existing ipwhitelist middleware. The changes include:

1. pkg/provider/kubernetes/ingress-nginx/annotations.go: I extended the ingressConfig struct to parse the whitelist-source-range annotation from the Ingress object.
2. pkg/provider/kubernetes/ingress-nginx/kubernetes.go: I added the applyWhitelistSourceRange function, which is called from applyMiddlewares. This function creates a new IPWhiteList middleware instance and attaches it to the router if the annotation is detected.
3. pkg/provider/kubernetes/ingress-nginx/annotations_test.go: I added a unit test to verify that the annotation is correctly parsed from a comma-separated string into a slice of strings.

This approach integrates the new annotation seamlessly into the existing provider logic.