[SPARK-15286][SQL] Make the output readable for EXPLAIN CREATE TABLE and DESC EXTENDED

#### What changes were proposed in this pull request?
Before this PR, the output of EXPLAIN of following SQL is like

```SQL
CREATE EXTERNAL TABLE extTable_with_partitions (key INT, value STRING)
PARTITIONED BY (ds STRING, hr STRING)
LOCATION '/private/var/folders/4b/sgmfldk15js406vk7lw5llzw0000gn/T/spark-b39a6185-8981-403b-a4aa-36fb2f4ca8a9'
```
``ExecutedCommand CreateTableCommand CatalogTable(`extTable_with_partitions`,CatalogTableType(EXTERNAL),CatalogStorageFormat(Some(/private/var/folders/4b/sgmfldk15js406vk7lw5llzw0000gn/T/spark-dd234718-e85d-4c5a-8353-8f1834ac0323),Some(org.apache.hadoop.mapred.TextInputFormat),Some(org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat),None,false,Map()),List(CatalogColumn(key,int,true,None), CatalogColumn(value,string,true,None), CatalogColumn(ds,string,true,None), CatalogColumn(hr,string,true,None)),List(ds, hr),List(),List(),-1,,1463026413544,-1,Map(),None,None,None), false``

After this PR, the output is like

```
ExecutedCommand
:  +- CreateTableCommand CatalogTable(
	Table:`extTable_with_partitions`
	Created:Thu Jun 02 21:30:54 PDT 2016
	Last Access:Wed Dec 31 15:59:59 PST 1969
	Type:EXTERNAL
	Schema:[`key` int, `value` string, `ds` string, `hr` string]
	Partition Columns:[`ds`, `hr`]
	Storage(Location:/private/var/folders/4b/sgmfldk15js406vk7lw5llzw0000gn/T/spark-a06083b8-8e88-4d07-9ff0-d6bd8d943ad3, InputFormat:org.apache.hadoop.mapred.TextInputFormat, OutputFormat:org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat)), false
```

This is also applicable to `DESC EXTENDED`. However, this does not have special handling for Data Source Tables. If needed, we need to move the logics of `DDLUtil`. Let me know if we should do it in this PR. Thanks! rxin liancheng

#### How was this patch tested?
Manual testing

Author: gatorsmile <gatorsmile@gmail.com>

Closes #13070 from gatorsmile/betterExplainCatalogTable.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/interface.scala

@@ -17,6 +17,7 @@
 
 package org.apache.spark.sql.catalyst.catalog
 
+import java.util.Date
 import javax.annotation.Nullable
 
 import org.apache.spark.sql.AnalysisException
@@ -48,7 +49,26 @@ case class CatalogStorageFormat(
     outputFormat: Option[String],
     serde: Option[String],
     compressed: Boolean,
-    serdeProperties: Map[String, String])
+    serdeProperties: Map[String, String]) {
+
+  override def toString: String = {
+    val serdePropsToString =
+      if (serdeProperties.nonEmpty) {
+        s"Properties: " + serdeProperties.map(p => p._1 + "=" + p._2).mkString("[", ", ", "]")
+      } else {
+        ""
+      }
+    val output =
+      Seq(locationUri.map("Location: " + _).getOrElse(""),
+        inputFormat.map("InputFormat: " + _).getOrElse(""),
+        outputFormat.map("OutputFormat: " + _).getOrElse(""),
+        if (compressed) "Compressed" else "",
+        serde.map("Serde: " + _).getOrElse(""),
+        serdePropsToString)
+    output.filter(_.nonEmpty).mkString("Storage(", ", ", ")")
+  }
+
+}
 
 object CatalogStorageFormat {
   /** Empty storage format for default values and copies. */
@@ -65,8 +85,18 @@ case class CatalogColumn(
     // as a string due to issues in converting Hive varchars to and from SparkSQL strings.
     @Nullable dataType: String,
     nullable: Boolean = true,
-    comment: Option[String] = None)
+    comment: Option[String] = None) {
+
+  override def toString: String = {
+    val output =
+      Seq(s"`$name`",
+        dataType,
+        if (!nullable) "NOT NULL" else "",
+        comment.map("(" + _ + ")").getOrElse(""))
+    output.filter(_.nonEmpty).mkString(" ")
+  }
 
+}
 
 /**
  * A partition (Hive style) defined in the catalog.
@@ -140,6 +170,32 @@ case class CatalogTable(
       locationUri, inputFormat, outputFormat, serde, compressed, serdeProperties))
   }
 
+  override def toString: String = {
+    val tableProperties = properties.map(p => p._1 + "=" + p._2).mkString("[", ", ", "]")
+    val partitionColumns = partitionColumnNames.map("`" + _ + "`").mkString("[", ", ", "]")
+    val sortColumns = sortColumnNames.map("`" + _ + "`").mkString("[", ", ", "]")
+    val bucketColumns = bucketColumnNames.map("`" + _ + "`").mkString("[", ", ", "]")
+
+    val output =
+      Seq(s"Table: ${identifier.quotedString}",
+        if (owner.nonEmpty) s"Owner: $owner" else "",
+        s"Created: ${new Date(createTime).toString}",
+        s"Last Access: ${new Date(lastAccessTime).toString}",
+        s"Type: ${tableType.name}",
+        if (schema.nonEmpty) s"Schema: ${schema.mkString("[", ", ", "]")}" else "",
+        if (partitionColumnNames.nonEmpty) s"Partition Columns: $partitionColumns" else "",
+        if (numBuckets != -1) s"Num Buckets: $numBuckets" else "",
+        if (bucketColumnNames.nonEmpty) s"Bucket Columns: $bucketColumns" else "",
+        if (sortColumnNames.nonEmpty) s"Sort Columns: $sortColumns" else "",
+        viewOriginalText.map("Original View: " + _).getOrElse(""),
+        viewText.map("View: " + _).getOrElse(""),
+        comment.map("Comment: " + _).getOrElse(""),
+        if (properties.nonEmpty) s"Properties: $tableProperties" else "",
+        s"$storage")
+
+    output.filter(_.nonEmpty).mkString("CatalogTable(\n\t", "\n\t", ")")
+  }
+
 }