Support struct field as indexed column

[dai-chen]

Description

1. To preserve the nested structure of subfields without flattening them, store the root StructType and retrieve fields by examining this structure.
2. When query rewrite, match struct column name with index expression.

TODO

To support any index expression in general, we need to analyze the given index expression string when create index or query rewrite. PoC is done here but requires more changes: #232. Will delay this until it's required.

Testing

I'm unable to check skipping index data in IT due to issue #233. Checked this manually by debugging IT:

curl "localhost:56262/flint_spark_catalog_default_nested_field_table_skipping_index/_search?pretty"
{
  "took" : 2,
  "timed_out" : false,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 2,
      "relation" : "eq"
    },
    "max_score" : 1.0,
    "hits" : [
      {
        "_index" : "flint_spark_catalog_default_nested_field_table_skipping_index",
        "_id" : "a66a1b7b17d545a9248d8a236b2d1c01a358b4a1",
        "_score" : 1.0,
        "_source" : {
          "file_path" : "file:///.../nested_field_table/part-00000-0f065a88-7d52-4e68-a8e3-d39e389221be-c000.json",
          "MinMax_struct_col.field2_0" : 123,
          "MinMax_struct_col.field2_1" : 456,
          "struct_col.field1.subfield" : [
            "value1",
            "value2"
          ]
        }
      },
      {
        "_index" : "flint_spark_catalog_default_nested_field_table_skipping_index",
        "_id" : "1ae12dc8aa4215668c0f4da81f5312ef21987568",
        "_score" : 1.0,
        "_source" : {
          "file_path" : "file:///.../nested_field_table/part-00000-f86ea4eb-19d7-4d60-bf76-61167537a2ad-c000.json",
          "MinMax_struct_col.field2_0" : 789,
          "MinMax_struct_col.field2_1" : 789,
          "struct_col.field1.subfield" : [
            "value3"
          ]
        }
      }
    ]
  }
}

Issues Resolved

#194

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