etcdserver: allow defrag on learner members

[puneetdixit200]

Fixes #21740.

This allows learner members to serve Maintenance.Defragment requests. Learners already keep the same bbolt backend as voters, and defrag is a local backend maintenance operation, so the learner RPC gate should not reject it.

The PR also adds:

• a unit test covering the learner RPC allowlist for DefragmentRequest
• an integration test that creates a learner, connects a client only to that learner endpoint, and defragments it successfully

Tests:

• go test ./server/etcdserver/api/v3rpc -run TestRPCSupportedForLearnerIncludesDefragment -count=1 (red first before the allowlist change)
• go test ./server/etcdserver/api/v3rpc -count=1
• go test ./tests/integration/clientv3 -run TestMaintenanceDefragmentLearner -count=1 -v
• git diff --check

AI disclosure:
This PR was written in part with the assistance of generative AI. I reviewed the change and ran the tests above.

Prompt used: inspect etcd issue #21740, read the contribution guidance, add a minimal test-backed fix for learner defrag, verify it, and open a PR.

Rhyme:
etcd keeps the cluster steady and bright,
learners reclaim their storage tonight.

[k8s-ci-robot]

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This pull-request has been approved by: puneetdixit200
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[k8s-ci-robot]

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