[BUG] ClassCastException in GpuStructsToJson

[andygrove]

Describe the bug

spark.conf.set("spark.rapids.sql.expression.StructsToJson", true)

val df = Seq(Array("a", "b", "c"), Array("d", "e", "f")).toDF("a").repartition(2)
val df2 = df.select(to_json(col("a")).alias("my_json"))
df2.show(false)

Fails with:

Caused by: java.lang.ClassCastException: org.apache.spark.sql.types.ArrayType cannot be cast to org.apache.spark.sql.types.StructType
  at org.apache.spark.sql.rapids.GpuStructsToJson.doColumnar(GpuStructsToJson.scala:86)

Steps/Code to reproduce bug
See above

Expected behavior

With the plugin disabled, Spark produces the following output.

+-------------+
|my_json      |
+-------------+
|["a","b","c"]|
|["d","e","f"]|
+-------------+

Environment details (please complete the following information)
N/A

Additional context

[andygrove]

The same issue exists for Map. StructsToJson supports Structs, Arrays, and Map, but our implementation currently assumes Structs.

GpuStructsToJson:

GpuCast.castStructToJsonString(input.getBase, child.dataType.asInstanceOf[StructType].fields, ...