[Variant] Carry Spark VariantType identity through Comet schema serialization

[peterxcli]

Problem

Comet cannot represent Spark VariantType in its protobuf, construct a logically marked Arrow Field for it, or recover the logical type from that Field on the JVM.

Parent: #5546
Depends on: #5547

Solution

• Append VARIANT to DataTypeId without renumbering existing values.
• Serialize Spark VariantType through Spark-version shims; Spark 3.x remains inert.
• Map it natively to physical Struct<value: Binary, metadata: Binary>.
• Attach ARROW:extension:name=arrow.parquet.variant to the outer Field.
• Make Utils.fromArrowField recognize only that explicit marker.
• Reuse CometStructVector; do not add a new vector class.
• Keep general expression, operator, and scan admission gates closed.

This task transports logical identity only. It must not yet admit Variant Parquet scans.

Tests

• Protobuf and Arrow Field mapping use [value, metadata].
• Only the explicit extension marker becomes Spark VariantType.
• An unmarked identical Struct remains StructType.
• General native Variant expressions remain unsupported.
• Spark 3.x compiles and behaves unchanged.

[peterxcli]

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