Ref #8480

Rationale for this change

Adds a builder method that configures VariantArrayBuilder to produce a shredded VariantArray at build() time.

What changes are included in this PR?

Add shredding_schema: Option field to VariantArrayBuilder
Add with_shredding(as_type: DataType) -> Self — rows are still appended unshredded via append_variant; the shredding pass runs once at build() by delegating to shred_variant

Are these changes tested?

unit tests

Are there any user-facing changes?

Yes, usage:

let arr = VariantArrayBuilder::new(3)
    .with_shredding(DataType::Int64)
    .tap(|b| { b.append_variant(Variant::Int64(42)); })
    .build(); // returns shredded VariantArray