[SPARK-15692][SQL] Improves the explain output of several physical plans by...

[SPARK-15692][SQL] Improves the explain output of several physical plans by displaying embedded logical plan in tree style

## What changes were proposed in this pull request?

Improves the explain output of several physical plans by displaying embedded logical plan in tree style

Some physical plan contains a embedded logical plan, for example, `cache tableName query` maps to:

```
case class CacheTableCommand(
    tableName: String,
    plan: Option[LogicalPlan],
    isLazy: Boolean)
  extends RunnableCommand
```

It is easier to read the explain output if we can display the `plan` in tree style.

**Before change:**

Everything is messed in one line.

```
scala> Seq((1,2)).toDF().createOrReplaceTempView("testView")
scala> spark.sql("cache table testView2 select * from testView").explain()
== Physical Plan ==
ExecutedCommand CacheTableCommand testView2, Some('Project [*]
+- 'UnresolvedRelation `testView`, None
), false
```

**After change:**

```
scala> spark.sql("cache table testView2 select * from testView").explain()
== Physical Plan ==
ExecutedCommand
:  +- CacheTableCommand testView2, false
:     :  +- 'Project [*]
:     :     +- 'UnresolvedRelation `testView`, None
```

## How was this patch tested?

Manual test.

Author: Sean Zhong <seanzhong@databricks.com>

Closes #13433 from clockfly/verbose_breakdown_3_2.