## 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.
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