Add index settings option in create statement

[dai-chen]

Description

1. Add new index_settings option. Please see doc: https://github.com/dai-chen/opensearch-spark/blob/add-index-settings-option/docs/index.md#create-index-options
2. Use a new FlintMetadata.indexSettings field to pass it to Flint client. Will refactor the Flint metadata and finalize spec in Refactoring Flint metadata and build index API #24

Example

spark-sql> CREATE INDEX orderkey_and_quantity
         > ON stream.lineitem_tiny (l_orderkey, l_quantity)
         > WITH (
         >   index_settings = '{"number_of_shards":9,"number_of_replicas":2}'
         > );

GET flint_stream_lineitem_tiny_orderkey_and_quantity_index/_settings
{
  "flint_stream_lineitem_tiny_orderkey_and_quantity_index": {
    "settings": {
      "index": {
        "creation_date": "1695680614547",
        "number_of_shards": "9",
        "number_of_replicas": "2",
        "uuid": "M-HlOt8yT1i7hKEm_GJmOw",
        "version": {
          "created": "136267827"
        },
        "provided_name": "flint_stream_lineitem_tiny_orderkey_and_quantity_index"
      }
    }
  }
}

Issues Resolved

#26

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

Approved by mistake