Add create, drop and refresh covering index SQL support

[dai-chen]

Description

1. Added SQL support for CREATE/DROP/REFRESH statement for covering index.
2. Updated user manual: https://github.com/dai-chen/opensearch-spark/blob/add-covering-index-sql-support/docs/index.md#covering-index
3. Refactor Flint SQL AST builder to separate and mix-in AST builder for different derived dataset

TODO

Will publish PR for SHOW/DESC statement separately for review convenience.

Example

Create covering index with auto refresh enabled and drop it when not needed:

spark-sql> CREATE INDEX orderkey_and_quantity 
... ON stream.lineitem_tiny (l_orderkey, l_quantity)
... WITH (auto_refresh = true);

spark-sql> DROP INDEX orderkey_and_quantity ON stream.lineitem_tiny;

OpenSearch index looks like:

GET flint_stream_lineitem_tiny_orderkey_and_quantity_index/_mapping
{
  "flint_stream_lineitem_tiny_orderkey_and_quantity_index": {
    "mappings": {
      "_meta": {
        "name": "orderkey_and_quantity",
        "source": "stream.lineitem_tiny",
        "kind": "covering",
        "indexedColumns": [
          {
            "columnType": "bigint",
            "columnName": "l_orderkey"
          },
          {
            "columnType": "float",
            "columnName": "l_quantity"
          }
        ]
      },
      "properties": {
        "l_orderkey": {
          "type": "long"
        },
        "l_quantity": {
          "type": "float"
        }
      }
    }
  }
}

GET flint_stream_lineitem_tiny_orderkey_and_quantity_index/_search
{
     ...
    "max_score": 1,
    "hits": [
      {
        "_index": "flint_stream_lineitem_tiny_orderkey_and_quantity_index",
        "_id": "a_hXqooBWdGHpYUCwr6x",
        "_score": 1,
        "_source": {
          "l_orderkey": 22828645,
          "l_quantity": 26
        }
      },
      {
        "_index": "flint_stream_lineitem_tiny_orderkey_and_quantity_index",
        "_id": "bPhXqooBWdGHpYUCwr6x",
        "_score": 1,
        "_source": {
          "l_orderkey": 59498081,
          "l_quantity": 16
        }
      },
      ...

Issues Resolved

#23

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[YANG-DB]

it would be very helpful if u can add a diagram showing the content of the skipping index and how it relates to the original table ...

[dai-chen]

Sure, I've created test for doc update. Will add doctest with examples in #28. Thanks!