go/writer: add variant parquet type

[jacobmarble]

Description:

Adds LogicalTypeVariant to the go/writer parquet package. Variant is a group of two required BYTE_ARRAY children (value and metadata) per the Parquet Variant spec, enabling connectors to emit JSON objects/arrays as native Variant columns instead of raw strings.

New options WithParquetSchemaObjectAsVariant() and WithParquetSchemaArrayAsVariant() let connectors opt in. A getVariantVal helper converts common Go types to variant.Value at write time.

Notes for reviewers:

• The first commit (a77f7de go/writer: column-oriented parquet write buffer) has its own PR at go/writer: column-oriented parquet write buffer #4485 — please review/merge that first. This PR contains only the top commit.
• LogicalTypeVariant maps to two physical parquet columns, so physicalColumnCount() was added to ParquetSchemaElement and columnChunkCount accounting was updated accordingly.