[Variant] Move VariantType from parquet-variant-compute to parquet-variant #11952

Is your feature request related to a problem or challenge?

Registering the canonical arrow.parquet.variant extension type currently requires a dependency on parquet-variant-compute, which also pulls in parquet-variant-json.

In apache/datafusion#26822, both datafusion-common and datafusion-expr need VariantType for schema validation, metadata handling, and the extension name. They do not need Variant compute kernels or JSON conversion for this registration, but must depend on those crates to access the type.

Describe the solution you'd like

Move VariantType into parquet-variant so schema-only consumers can depend on that crate directly. Its current implementation only needs arrow-schema, which parquet-variant already depends on.

Keep the existing import path available through a re-export in parquet-variant-compute:

pub use parquet_variant::VariantType;

Describe alternatives you've considered

Additional context

VariantType was introduced alongside the VariantArray extension-type conversion in #8392. Improving its storage-schema validation is tracked separately in #11938; this proposal only changes where the type is exposed.