Add EndpointSlice consumer helper functions

[mbergo]

This commit adds helper functions for EndpointSlice consumers to make it easier to transition from Endpoints to EndpointSlices. The new package provides:

1. EndpointSliceConsumer - Core component that tracks EndpointSlices and provides a unified view of endpoints for a service
2. EndpointSliceInformer - Informer-like interface for EndpointSlices
3. EndpointSliceLister - Lister-like interface for EndpointSlices

These helpers handle the complexity of merging multiple slices for the same service and deduplicating endpoints that might appear in multiple slices.

Benefits:

• Easier migration from Endpoints to EndpointSlices with familiar interfaces
• Simplified handling of multiple slices without manual merging and deduplication
• Improved performance by leveraging the scalability of the EndpointSlice API
• Consistent view of endpoints even as they move between slices

Fixes #124777

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[k8s-ci-robot]

Hi @mbergo. Thanks for your PR.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: mbergo
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[mbergo]

/cc @kubernetes/sig-network-pr-reviews

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[k8s-ci-robot]

@mbergo: Reiterating the mentions to trigger a notification:
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/cc @kubernetes/sig-network-pr-reviews

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[mbergo]

maybe like this. cc: @kubernetes/sig-network-pr-reviews

[k8s-ci-robot]

@mbergo: Reiterating the mentions to trigger a notification:
@kubernetes/sig-network-pr-reviews

In response to this:

maybe like this. cc: @kubernetes/sig-network-pr-reviews

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