[BUG] The schema order is not fixed due to hashmap

[xinyual]

What is the bug? Currently, since we use hashmap in https://github.com/opensearch-project/sql/blob/f1e3b5baa5ea17ff7bf5f8b21e3d3d9f53be072f/opensearch/src/main/java/org/opensearch/sql/opensearch/request/system/OpenSearchDescribeIndexRequest.java#L100 The schema's order is not fixed and can be influenced by external condition like random seeds. It can cause future block in IT.

What is the expected behavior? Replace by linkedHashMap.

What is your host/environment?

• OS: [e.g. iOS]
• Version [e.g. 22]
• Plugins

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