[SPARK-20487][SQL] Display `serde` for `HiveTableScan` node in explained plan

## What changes were proposed in this pull request?

This was a suggestion by rxin at https://github.com/apache/spark/pull/17780#issuecomment-298073408

## How was this patch tested?

- modified existing unit test
- manual testing:

```
scala> hc.sql(" SELECT * FROM tejasp_bucketed_partitioned_1  where name = ''  ").explain(true)
== Parsed Logical Plan ==
'Project [*]
+- 'Filter ('name = )
   +- 'UnresolvedRelation `tejasp_bucketed_partitioned_1`

== Analyzed Logical Plan ==
user_id: bigint, name: string, ds: string
Project [user_id#24L, name#25, ds#26]
+- Filter (name#25 = )
   +- SubqueryAlias tejasp_bucketed_partitioned_1
      +- CatalogRelation `default`.`tejasp_bucketed_partitioned_1`, org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe, [user_id#24L, name#25], [ds#26]

== Optimized Logical Plan ==
Filter (isnotnull(name#25) && (name#25 = ))
+- CatalogRelation `default`.`tejasp_bucketed_partitioned_1`, org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe, [user_id#24L, name#25], [ds#26]

== Physical Plan ==
*Filter (isnotnull(name#25) && (name#25 = ))
+- HiveTableScan [user_id#24L, name#25, ds#26], CatalogRelation `default`.`tejasp_bucketed_partitioned_1`, org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe, [user_id#24L, name#25], [ds#26]
```

Author: Tejas Patil <tejasp@fb.com>

Closes #17806 from tejasapatil/add_serde.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala

@@ -444,7 +444,11 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
     case None => Nil
     case Some(null) => Nil
     case Some(any) => any :: Nil
-    case table: CatalogTable => table.identifier :: Nil
+    case table: CatalogTable =>
+      table.storage.serde match {
+        case Some(serde) => table.identifier :: serde :: Nil
+        case _ => table.identifier :: Nil
+      }
     case other => other :: Nil
   }.mkString(", ")
 

sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveExplainSuite.scala

@@ -43,7 +43,9 @@ class HiveExplainSuite extends QueryTest with SQLTestUtils with TestHiveSingleto
 
   test("explain extended command") {
     checkKeywordsExist(sql(" explain   select * from src where key=123 "),
-                   "== Physical Plan ==")
+                   "== Physical Plan ==",
+                   "org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe")
+
     checkKeywordsNotExist(sql(" explain   select * from src where key=123 "),
                    "== Parsed Logical Plan ==",
                    "== Analyzed Logical Plan ==",