feat(zrpc): migrate kube resolver from Endpoints to EndpointSlice API

[soasurs]

Motivation

Kubernetes is gradually deprecating the legacy core/v1 Endpoints API in favor of the newer discovery/v1 EndpointSlice API.
Compared with Endpoints, EndpointSlice objects:

• Scale horizontally—each slice covers a subset of endpoints, avoiding the single-object size limit.
• Reduce API-server churn—only slices that change are updated.
• Provide topology and readiness metadata that can be leveraged for smarter routing.

To keep go-zero up-to-date with upstream best practices and to prevent future breakage as clusters disable the old API, we need to adopt EndpointSlice.

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What this PR does

1. Event Path Migration

   • Replaces all core/v1 Endpoints types with discovery/v1 EndpointSlice in the kube resolver.
   • Adjusts event-handler logic to iterate over EndpointSlice.Endpoints[].Addresses[].

2. Informer Updates

   • Swaps the informer factory from Core().V1().Endpoints() to Discovery().V1().EndpointSlices().
   • Uses the recommended label selector kubernetes.io/service-name=<svc>.

3. Initial Sync & Port Resolution

   • Lists existing EndpointSlice objects for the target service at startup.
   • Resolves svc.Port from the first available slice port when the user omits it.

4. Constants & Clean-ups

   • Renames nameSelector → serviceSelector for clarity.
   • Adds explicit error reporting when no matching slices are found.

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Impact

• Transparent to callers – existing zrpc resolver API remains unchanged.
• Better scalability & performance – large services no longer hit the 1000-endpoint hard limit.
• Future-proof – compatible with clusters that have disabled the legacy Endpoints API.
• RBAC change required – controllers using this resolver must be granted get, list, watch on EndpointSlices (discovery.k8s.io group).

No other functional regressions are expected. All unit tests pass against Kubernetes ≥1.21 (the minimum version that GA-supports EndpointSlice).

[codecov[bot]]

Codecov Report

:x: Patch coverage is 47.82609% with 24 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
zrpc/resolver/internal/kubebuilder.go	0.00%	24 Missing :warning:

:loudspeaker: Thoughts on this report? Let us know!

[kevwan]

Thanks for your contribution!

This PR will be merged in v1.9.0