### What changes were proposed in this pull request?
When `spark.sql.caseSensitive` is `false` (by default), check that there are not duplicate column names on the same level (top level or nested levels) in reading from in-built datasources Parquet, ORC, Avro and JSON. If such duplicate columns exist, throw the exception:
```
org.apache.spark.sql.AnalysisException: Found duplicate column(s) in the data schema:
```
### Why are the changes needed?
To make handling of duplicate nested columns is similar to handling of duplicate top-level columns i. e. output the same error when `spark.sql.caseSensitive` is `false`:
```Scala
org.apache.spark.sql.AnalysisException: Found duplicate column(s) in the data schema: `camelcase`
```
Checking of top-level duplicates was introduced by https://github.com/apache/spark/pull/17758.
### Does this PR introduce _any_ user-facing change?
Yes. For the example from SPARK-32431:
ORC:
```scala
java.io.IOException: Error reading file: file:/private/var/folders/p3/dfs6mf655d7fnjrsjvldh0tc0000gn/T/spark-c02c2f9a-0cdc-4859-94fc-b9c809ca58b1/part-00001-63e8c3f0-7131-4ec9-be02-30b3fdd276f4-c000.snappy.orc
at org.apache.orc.impl.RecordReaderImpl.nextBatch(RecordReaderImpl.java:1329)
at org.apache.orc.mapreduce.OrcMapreduceRecordReader.ensureBatch(OrcMapreduceRecordReader.java:78)
...
Caused by: java.io.EOFException: Read past end of RLE integer from compressed stream Stream for column 3 kind DATA position: 6 length: 6 range: 0 offset: 12 limit: 12 range 0 = 0 to 6 uncompressed: 3 to 3
at org.apache.orc.impl.RunLengthIntegerReaderV2.readValues(RunLengthIntegerReaderV2.java:61)
at org.apache.orc.impl.RunLengthIntegerReaderV2.next(RunLengthIntegerReaderV2.java:323)
```
JSON:
```scala
+------------+
|StructColumn|
+------------+
| [,,]|
+------------+
```
Parquet:
```scala
+------------+
|StructColumn|
+------------+
| [0,, 1]|
+------------+
```
Avro:
```scala
+------------+
|StructColumn|
+------------+
| [,,]|
+------------+
```
After the changes, Parquet, ORC, JSON and Avro output the same error:
```scala
Found duplicate column(s) in the data schema: `camelcase`;
org.apache.spark.sql.AnalysisException: Found duplicate column(s) in the data schema: `camelcase`;
at org.apache.spark.sql.util.SchemaUtils$.checkColumnNameDuplication(SchemaUtils.scala:112)
at org.apache.spark.sql.util.SchemaUtils$.checkSchemaColumnNameDuplication(SchemaUtils.scala:51)
at org.apache.spark.sql.util.SchemaUtils$.checkSchemaColumnNameDuplication(SchemaUtils.scala:67)
```
### How was this patch tested?
Run modified test suites:
```
$ build/sbt "sql/test:testOnly org.apache.spark.sql.FileBasedDataSourceSuite"
$ build/sbt "avro/test:testOnly org.apache.spark.sql.avro.*"
```
and added new UT to `SchemaUtilsSuite`.
Closes#29234 from MaxGekk/nested-case-insensitive-column.
Authored-by: Max Gekk <max.gekk@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
99a855575c