feat(discov): expose etcd AutoSyncInterval in EtcdConf

[Vierblatt]

Summary

Adds AutoSyncInterval field to EtcdConf, allowing users to configure the etcd client auto sync interval. When set (>0), it overrides the default via per-cluster registration, matching the existing pattern of RegisterAccount and RegisterTLS.

Etcd:
  Hosts:
    - 127.0.0.1:2479
  Key: device.rpc
  AutoSyncInterval: 0s  # disables auto-sync

Motivation

Fixes #5619. go-zero's etcd discovery creates the etcd client internally, so users cannot control AutoSyncInterval today. This matters when applications should keep using the configured Etcd.Hosts, such as an etcd LB / VIP / proxy (related: #5005).

Test plan

• All existing discov tests pass
• All existing zrpc tests pass
• Build successful
• Field is optional (zero value = use default behavior)

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
core/discov/internal/registry.go	0.00%	1 Missing and 1 partial ⚠️
zrpc/internal/rpcpubserver.go	0.00%	1 Missing and 1 partial ⚠️

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