…conf

## What changes were proposed in this pull request?

This is an effort to reduce the difference between Hive and Spark. Spark supports case-sensitivity in columns. Especially, for Struct types, with `spark.sql.caseSensitive=true`, the following is supported.

```scala
scala> sql("select named_struct('a', 1, 'A', 2).a").show
+--------------------------+
|named_struct(a, 1, A, 2).a|
+--------------------------+
|                         1|
+--------------------------+

scala> sql("select named_struct('a', 1, 'A', 2).A").show
+--------------------------+
|named_struct(a, 1, A, 2).A|
+--------------------------+
|                         2|
+--------------------------+
```

And vice versa, with `spark.sql.caseSensitive=false`, the following is supported.
```scala
scala> sql("select named_struct('a', 1).A, named_struct('A', 1).a").show
+--------------------+--------------------+
|named_struct(a, 1).A|named_struct(A, 1).a|
+--------------------+--------------------+
|                   1|                   1|
+--------------------+--------------------+
```

However, types are considered different. For example, SET operations fail.
```scala
scala> sql("SELECT named_struct('a',1) union all (select named_struct('A',2))").show
org.apache.spark.sql.AnalysisException: Union can only be performed on tables with the compatible column types. struct<A:int> <> struct<a:int> at the first column of the second table;;
'Union
:- Project [named_struct(a, 1) AS named_struct(a, 1)#57]
:  +- OneRowRelation$
+- Project [named_struct(A, 2) AS named_struct(A, 2)#58]
   +- OneRowRelation$
```

This PR aims to support case-insensitive type equality. For example, in Set operation, the above operation succeed when `spark.sql.caseSensitive=false`.

```scala
scala> sql("SELECT named_struct('a',1) union all (select named_struct('A',2))").show
+------------------+
|named_struct(a, 1)|
+------------------+
|               [1]|
|               [2]|
+------------------+
```

## How was this patch tested?

Pass the Jenkins with a newly add test case.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes apache#18460 from dongjoon-hyun/SPARK-21247.

…ntir#57)