[SPARK-14251][SQL] Add SQL command for printing out generated code for debugging

## What changes were proposed in this pull request?

This PR implements `EXPLAIN CODEGEN` SQL command which returns generated codes like `debugCodegen`. In `spark-shell`, we don't need to `import debug` module. In `spark-sql`, we can use this SQL command now.

**Before**
```
scala> import org.apache.spark.sql.execution.debug._
scala> sql("select 'a' as a group by 1").debugCodegen()
Found 2 WholeStageCodegen subtrees.
== Subtree 1 / 2 ==
...

Generated code:
...

== Subtree 2 / 2 ==
...

Generated code:
...
```

**After**
```
scala> sql("explain extended codegen select 'a' as a group by 1").collect().foreach(println)
[Found 2 WholeStageCodegen subtrees.]
[== Subtree 1 / 2 ==]
...
[]
[Generated code:]
...
[]
[== Subtree 2 / 2 ==]
...
[]
[Generated code:]
...
```

## How was this patch tested?

Pass the Jenkins tests (including new testcases)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12099 from dongjoon-hyun/SPARK-14251.