[Arrow] Move parquet-variant-compute StructArrayBuilder to arrow-array

### Is your feature request related to a problem or challenge?

`parquet-variant-compute` contains a local `StructArrayBuilder` for assembling a `StructArray` from completed child arrays, fields, and an optional null buffer.

This is a general Arrow array utility rather than Variant-specific functionality.

### Describe the solution you'd like

Add the utility, or equivalent functionality, to `arrow-array` and replace the local implementation in `parquet-variant-compute`.

### Additional context

Current TODO: https://github.com/apache/arrow-rs/blob/main/parquet-variant-compute/src/variant_array.rs#L927