[SPARK-16429][SQL] Include `StringType` columns in `describe()`

## What changes were proposed in this pull request?

Currently, Spark `describe` supports `StringType`. However, `describe()` returns a dataset for only all numeric columns. This PR aims to include `StringType` columns in `describe()`, `describe` without argument.

**Background**
```scala
scala> spark.read.json("examples/src/main/resources/people.json").describe("age", "name").show()
+-------+------------------+-------+
|summary|               age|   name|
+-------+------------------+-------+
|  count|                 2|      3|
|   mean|              24.5|   null|
| stddev|7.7781745930520225|   null|
|    min|                19|   Andy|
|    max|                30|Michael|
+-------+------------------+-------+
```

**Before**
```scala
scala> spark.read.json("examples/src/main/resources/people.json").describe().show()
+-------+------------------+
|summary|               age|
+-------+------------------+
|  count|                 2|
|   mean|              24.5|
| stddev|7.7781745930520225|
|    min|                19|
|    max|                30|
+-------+------------------+
```

**After**
```scala
scala> spark.read.json("examples/src/main/resources/people.json").describe().show()
+-------+------------------+-------+
|summary|               age|   name|
+-------+------------------+-------+
|  count|                 2|      3|
|   mean|              24.5|   null|
| stddev|7.7781745930520225|   null|
|    min|                19|   Andy|
|    max|                30|Michael|
+-------+------------------+-------+
```

## How was this patch tested?

Pass the Jenkins with a update testcase.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #14095 from dongjoon-hyun/SPARK-16429.