Add support to cast from UnionArray (#9544)

# Which issue does this PR close?

- Closes #9225

# Rationale for this change

This PR adds native support for casting Union arrays in the cast kernel.
Previously, `can_cast_types` and `cast_with_options` had no handling for
union types at all

Author: friendlymatthew

arrow-cast/src/cast/mod.rs

@@ -43,13 +43,15 @@ mod list;
 mod map;
 mod run_array;
 mod string;
+mod union;
 
 use crate::cast::decimal::*;
 use crate::cast::dictionary::*;
 use crate::cast::list::*;
 use crate::cast::map::*;
 use crate::cast::run_array::*;
 use crate::cast::string::*;
+pub use crate::cast::union::*;
 
 use arrow_buffer::IntervalMonthDayNano;
 use arrow_data::ByteView;
@@ -108,6 +110,8 @@ pub fn can_cast_types(from_type: &DataType, to_type: &DataType) -> bool {
             can_cast_types(from_value_type, to_value_type)
         }
         (Dictionary(_, value_type), _) => can_cast_types(value_type, to_type),
+        (Union(fields, _), _) => union::resolve_child_array(fields, to_type).is_some(),
+        (_, Union(_, _)) => false,
         (RunEndEncoded(_, value_type), _) => can_cast_types(value_type.data_type(), to_type),
         (_, RunEndEncoded(_, value_type)) => can_cast_types(from_type, value_type.data_type()),
         (_, Dictionary(_, value_type)) => can_cast_types(from_type, value_type),
@@ -230,7 +234,6 @@ pub fn can_cast_types(from_type: &DataType, to_type: &DataType) -> bool {
         }
         (Struct(_), _) => false,
         (_, Struct(_)) => false,
-
         (_, Boolean) => from_type.is_integer() || from_type.is_floating() || from_type.is_string(),
         (Boolean, _) => to_type.is_integer() || to_type.is_floating() || to_type.is_string(),
 
@@ -781,6 +784,14 @@ pub fn cast_with_options(
                 ))),
             }
         }
+        (Union(_, _), _) => union_extract_by_type(
+            array.as_any().downcast_ref::<UnionArray>().unwrap(),
+            to_type,
+            cast_options,
+        ),
+        (_, Union(_, _)) => Err(ArrowError::CastError(format!(
+            "Casting from {from_type} to {to_type} not supported"
+        ))),
         (Dictionary(index_type, _), _) => match **index_type {
             Int8 => dictionary_cast::<Int8Type>(array, to_type, cast_options),
             Int16 => dictionary_cast::<Int16Type>(array, to_type, cast_options),