feat: add graceful stop annotation

[Bowser1704]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Adds a new Service annotation service.beta.kubernetes.io/alibaba-cloud-loadbalancer-graceful-shutdown.

When enabled for ENI backends, terminating EndpointSlice endpoints that are still serving are kept in CLB/NLB backend server groups with weight 0, allowing existing connections to drain before the backend is removed after pod termination completes.

This PR also adds unit tests and E2E coverage for CLB and NLB graceful shutdown behavior.

Which issue(s) this PR fixes:

Fixes #485

Special notes for your reviewer:

This feature applies to EndpointSlice-based reconciliation and ENI backend mode.

graceful-shutdown relies on backend weight updates to set terminating-but-serving backends to weight 0. If users also enable ignore-weight-update, the backend is kept in the server group but
the weight is not changed. In that case, users should enable connection-drain to reduce connection interruption risk when the backend is finally removed.

Does this PR introduce a user-facing change?

Add Service annotation `service.beta.kubernetes.io/alibaba-cloud-loadbalancer-graceful-shutdown` to support graceful backend draining for terminating ENI pods.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Bowser1704
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