[Variant] Support extracting struct fields as Variant using ExtensionType

[codephage2020]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

arrow-rs/parquet-variant-compute/src/variant_get.rs

Lines 273 to 276 in 5ba4515

	/// TODO: How would a caller request a struct or list type where the fields/elements can be any
	/// variant? Caller can pass None as the requested type to fetch a specific path, but it would
	/// quickly become annoying (and inefficient) to call `variant_get` for each leaf value in a struct or
	/// list and then try to assemble the results.

Given data like:

{"user": {"id": 123, "metadata": {"tags": ["a"], "score": 95.5}}}

Users want to extract user with:

• id as Int32
• metadata as Variant (not expanded)

Currently, this requires multiple variant_get calls and manual StructArray assembly.

Describe the solution you'd like

Use ExtensionType to mark fields that should remain as Variant:

let field = Field::new("user", DataType::Struct(Fields::from(vec![
    Field::new("id", DataType::Int32, true),
    Field::new("metadata", ..., true).with_extension_type(VariantType),  // Keep as Variant
])), true);

let result = variant_get(&array, GetOptions::new().with_as_type(Some(Arc::new(field))))?;

[sdf-jkl]

This seems related - #8154