[SPARK-16543][SQL] Rename the columns of `SHOW PARTITION/COLUMNS` commands

## What changes were proposed in this pull request?

This PR changes the name of columns returned by `SHOW PARTITION` and `SHOW COLUMNS` commands. Currently, both commands uses `result` as a column name.

**Comparison: Column Name**

Command|Spark(Before)|Spark(After)|Hive
----------|--------------|------------|-----
SHOW PARTITIONS|result|partition|partition
SHOW COLUMNS|result|col_name|field

Note that Spark/Hive uses `col_name` in `DESC TABLES`. So, this PR chooses `col_name` for consistency among Spark commands.

**Before**
```scala
scala> sql("show partitions p").show()
+------+
|result|
+------+
|   b=2|
+------+

scala> sql("show columns in p").show()
+------+
|result|
+------+
|     a|
|     b|
+------+
```

**After**
```scala
scala> sql("show partitions p").show
+---------+
|partition|
+---------+
|      b=2|
+---------+

scala> sql("show columns in p").show
+--------+
|col_name|
+--------+
|       a|
|       b|
+--------+
```

## How was this patch tested?

Manual.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #14199 from dongjoon-hyun/SPARK-16543.

sql/core/src/main/scala/org/apache/spark/sql/execution/command/tables.scala

@@ -622,9 +622,8 @@ case class ShowTablePropertiesCommand(table: TableIdentifier, propertyKey: Optio
  * }}}
  */
 case class ShowColumnsCommand(table: TableIdentifier) extends RunnableCommand {
-  // The result of SHOW COLUMNS has one column called 'result'
   override val output: Seq[Attribute] = {
-    AttributeReference("result", StringType, nullable = false)() :: Nil
+    AttributeReference("col_name", StringType, nullable = false)() :: Nil
   }
 
   override def run(sparkSession: SparkSession): Seq[Row] = {
@@ -652,9 +651,8 @@ case class ShowColumnsCommand(table: TableIdentifier) extends RunnableCommand {
 case class ShowPartitionsCommand(
     table: TableIdentifier,
     spec: Option[TablePartitionSpec]) extends RunnableCommand {
-  // The result of SHOW PARTITIONS has one column called 'result'
   override val output: Seq[Attribute] = {
-    AttributeReference("result", StringType, nullable = false)() :: Nil
+    AttributeReference("partition", StringType, nullable = false)() :: Nil
   }
 
   private def getPartName(spec: TablePartitionSpec, partColNames: Seq[String]): String = {