Expose Arrow `Struct` columns as JDBC `STRUCT` metadata in the Flight SQL JDBC driver.

### Describe the enhancement requested

Please expose Arrow `Struct` columns as JDBC `STRUCT` metadata in the Flight SQL JDBC driver.

Today, Arrow `Struct` is mapped to `java.sql.Types.JAVA_OBJECT`, which makes
`ResultSetMetaData` report:
- `getColumnType(...) == Types.JAVA_OBJECT`
- `getColumnTypeName(...) == "JAVA_OBJECT"`

This hides the fact that the column is structurally a JDBC `STRUCT`, even though the driver
already has struct-specific accessor support.

## Why this matters

JDBC clients often rely on `ResultSetMetaData` to understand result schemas before reading
rows. When `STRUCT` columns are reported as `JAVA_OBJECT`, clients cannot distinguish:
- struct/object-like columns
- generic opaque objects

This makes schema inspection, BI tooling, ORM adapters, and generic JDBC consumers harder.

## Current upstream evidence

In
`flight/flight-sql-jdbc-core/src/main/java/org/apache/arrow/driver/jdbc/utils/SqlTypes.java`:

- `ArrowType.Struct` maps to `Types.JAVA_OBJECT`

In `flight/flight-sql-jdbc-core/src/main/java/org/apache/arrow/driver/jdbc/accessor/impl/comple
x/ArrowFlightJdbcStructVectorAccessor.java`:

- `getStruct()` is already implemented
- but `getObjectClass()` returns `Map.class`
- `getObject()` returns a `Map`

So the data path knows about structs, but metadata does not expose them as JDBC `STRUCT`.

## Expected behavior

For Arrow `Struct` columns:
- `ResultSetMetaData.getColumnType(...)` should return `java.sql.Types.STRUCT`
- `ResultSetMetaData.getColumnTypeName(...)` should return `STRUCT`

Optionally, `getObject()` / object-class behavior could also be aligned with JDBC `Struct`, but
the most important gap is metadata.

## Actual behavior

Arrow `Struct` columns are exposed as:
- `Types.JAVA_OBJECT`
- `"JAVA_OBJECT"`

## Version

Observed with:
- Arrow Flight SQL JDBC driver `18.2.0`

Confirmed in current upstream code and latest published artifact line:
- `19.0.0`

flight/flight-sql-jdbc-core/src/main/java/org/apache/arrow/driver/jdbc/utils/SqlTypes.java から始めます。ここでは ArrowType.Struct が Types.JAVA_OBJECT にマッピングされているため、関連する ArrowFlightJdbcStructVectorAccessor.java の実装を確認してください。Struct 列が JDBC STRUCT を報告するようにメタデータマッピングを更新し、それらの列について ResultSetMetaData が STRUCT と "STRUCT" を返すことを確認してください。

索引モデルが issue の本文から書いたものです。