Support Union types in StructBuilder

[V0ldek]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I'm trying to use StructBuilder to create a struct whose one field is of a Union type. This is currently unsupported, see this small repro (crate version v52.2.0):

use arrow::{
    array::StructBuilder,
    datatypes::{DataType, Field, Fields, UnionFields, UnionMode},
};

fn main() {
    let _builder = StructBuilder::from_fields(
        get_struct_fields(),
        3,
    );
}

fn get_struct_fields() -> Fields {
    use arrow::datatypes::{DataType, Field, Fields};
    Fields::from(vec![
        Field::new("name", DataType::Utf8, false),
        Field::new("value", get_union_type(), false),
    ])
}

fn get_union_type() -> DataType {
    let fields = vec![
        Field::new("Integer", DataType::Int32, false),
        Field::new("Float", DataType::Float32, false),
    ];
    DataType::Union(UnionFields::new(vec![0, 1], fields), UnionMode::Sparse)
}

This results in a panic from builder/struct_builder.rs:278:14:

Data type Union([(0, Field { name: "Integer", data_type: Int32, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), (1, Field { name: "Float", data_type: Float32, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} })], Sparse) is not currently supported

The particular layout of the union doesn't matter, neither does whether it's sparse or dense.

Describe the solution you'd like
I'd like to use StructBuilder with a Union field the same way it works with other field types. In particular, I should be able to retrieve a UnionBuilder for a field:

    let mut builder = StructBuilder::from_fields(
        get_struct_fields(),
        3,
    );
    let union_field = builder.field_builder::<UnionBuilder>(1);

Describe alternatives you've considered
I don't actually know how to work around this. It'd be appreciated if someone could show me how to build a struct array with a schema like this without using the builder, so I can unblock myself.

Additional context
As far as I understand the main issue is that UnionBuilder does not implement the ArrayBuilder trait, which is required for the dynamic API and internals of StructBuilder. This is because ArrayBuilder requires a finish(&mut self) -> ArrayRef, and the UnionBuilder has a build(self) -> Result<UnionArray, ArrowError> (consumes the builder AND can fail).

Changing self to &mut self shouldn't be too hard, but the Result semantics are rather hard. I'm not sure what the implementation path would be here.