optimize: metadata discovery support for zookeeper

[YoWuwuuuw]

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• I have registered the PR changes.

Ⅰ. Describe what this PR did

Enhance the Zookeeper registry center with metadata registration and discovery capabilities based on branch 'gsoc-2025-meta-registry'.

1. Register and write metadata: In register(ServiceInstance), the metadata is serialized into a string and written to the ZK temporary node data. The node path remains .../registry/zk/{cluster}/{ip:port}.

2. Lookup/listen and backfill metadata: In lookup and child node change callbacks, getData() reads the node data, parses it into a Map<String,String>, constructs it using ServiceInstance.fromStringMap(address, metadata), and sends the metadata back to the calling side.

3. Prevent metadata loss during reconnection recover(): A new REGISTERED_PATH_DATA_MAP cache is added for path→data. When recover() rebuilds the temporary node, it uses the cached data to write back, ensuring metadata consistency.

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

1. Compatibility: If getData() fails or the node has no data, it will fall back to the address instance InetSocketAddress only.
2. Serialization: Uses lightweight line-based key=value\n text, with zero dependencies;