refactor(common): (WIP/Preview) optional name for struct data type

[xiangjinwu]

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What's changed and what's your intention?

Motivation

Given the following column with multiple levels of nested struct:

a struct<     -- let's call this struct A
  a struct<   -- struct AA
    a struct< -- struct AAA
      a int2,
      b int4
    >,
    b struct< -- struct AAB
      a int8,
      b decimal
    >
  >,
  b struct<   -- struct AB
    a struct< -- struct ABA
      a float4,
      b float8
    >,
    b struct< -- struct ABB
      a varchar,
      b bytea
    >
  >
>

The current ColumnDesc contains duplicate information:

ColumnDesc { -- column a of type A
  column_type: Struct:::field_types [
    Struct::field_types [
      Struct::field_types [
        Int16,
        Int32,
      ],
      Struct::field_types [
        Int64,
        Decimal,
      ],
    ],
    Struct::field_types [
      Struct::field_types [
        Float32,
        Float64,
      ],
      Struct::field_types [
        Varchar,
        Bytea,
      ],
    ],
  ],
  field_descs: [
    ColumnDesc { -- field (a).a of type AA
      column_type: Struct::field_types [
        Struct::field_types [
          Int16,
          Int32,
        ],
        Struct::field_types [
          Int64,
          Decimal,
        ],
      ],
      field_descs: [
        ColumnDesc { -- field (a).a.a of type AAA
          column_type: Struct::field_types [
            Int16,
            Int32,
          ],
          field_descs: [
            ColumnDesc { -- field (a).a.a.a
              column_type: Int16,
            },
            ColumnDesc { -- field (a).a.a.b
              column_type: Int32,
            },
          ],
        },
        ColumnDesc { -- field (a).a.b of type AAB
          column_type: Struct::field_types [
            Int64,
            Decimal,
          ],
          field_descs: [
            ColumnDesc { -- field (a).a.b.a
              column_type: Int64,
            },
            ColumnDesc { -- field (a).a.b.b
              column_type: Decimal,
            },
          ],
        },
      ],
    },
    ColumnDesc { -- field (a).b of type AB
      column_type: Struct::field_types [
        Struct::field_types [
          Float32,
          Float64,
        ],
        Struct::field_types [
          Varchar,
          Bytea,
        ],
      ],
      field_descs: [
        ColumnDesc { -- field (a).b.a of type ABA
          column_type: Struct::field_types [
            Float32,
            Float64,
          ],
          field_descs: [
            ColumnDesc { -- field (a).a.a.a
              column_type: Float32,
            },
            ColumnDesc { -- field (a).a.a.b
              column_type: Float64,
            },
          ],
        },
        ColumnDesc { -- field (a).b.b of type ABB
          column_type: Struct::field_types [
            Varchar,
            Bytea,
          ],
          field_descs: [
            ColumnDesc { -- field (a).a.b.a
              column_type: Varchar,
            },
            ColumnDesc { -- field (a).a.b.b
              column_type: Bytea,
            },
          ],
        },
      ],
    },
  ],
}

The ultimate goal is to deprecated field_descs and keep column_type. So column_type need the ability to hold the optional name of struct type.

Checklist

• I have written necessary rustdoc comments
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• I have added test labels as necessary. See details.
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• My PR changes performance-critical code. (Please run macro/micro-benchmarks and show the results.)

• My PR contains critical fixes that are necessary to be merged into the latest release. (Please check out the details)

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Release note

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