[SPARK-15792][SQL] Allows operator to change the verbosity in explain output

## What changes were proposed in this pull request?

This PR allows customization of verbosity in explain output. After change, `dataframe.explain()` and `dataframe.explain(true)` has different verbosity output for physical plan.

Currently, this PR only enables verbosity string for operator `HashAggregateExec` and `SortAggregateExec`. We will gradually enable verbosity string for more operators in future.

**Less verbose mode:** dataframe.explain(extended = false)

`output=[count(a)#85L]` is **NOT** displayed for HashAggregate.

```
scala> Seq((1,2,3)).toDF("a", "b", "c").createTempView("df2")
scala> spark.sql("select count(a) from df2").explain()
== Physical Plan ==
*HashAggregate(key=[], functions=[count(1)])
+- Exchange SinglePartition
   +- *HashAggregate(key=[], functions=[partial_count(1)])
      +- LocalTableScan
```

**Verbose mode:** dataframe.explain(extended = true)

`output=[count(a)#85L]` is displayed for HashAggregate.

```
scala> spark.sql("select count(a) from df2").explain(true)  // "output=[count(a)#85L]" is added
...
== Physical Plan ==
*HashAggregate(key=[], functions=[count(1)], output=[count(a)#85L])
+- Exchange SinglePartition
   +- *HashAggregate(key=[], functions=[partial_count(1)], output=[count#87L])
      +- LocalTableScan
```

## How was this patch tested?

Manual test.

Author: Sean Zhong <seanzhong@databricks.com>

Closes #13535 from clockfly/verbose_breakdown_2.