[SPARK-12164][SQL] Decode the encoded values and then display

Based on the suggestions from marmbrus cloud-fan in https://github.com/apache/spark/pull/10165 , this PR is to print the decoded values(user objects) in `Dataset.show`
```scala
    implicit val kryoEncoder = Encoders.kryo[KryoClassData]
    val ds = Seq(KryoClassData("a", 1), KryoClassData("b", 2), KryoClassData("c", 3)).toDS()
    ds.show(20, false);
```
The current output is like
```
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|value                                                                                                                                                                                 |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|[1, 0, 111, 114, 103, 46, 97, 112, 97, 99, 104, 101, 46, 115, 112, 97, 114, 107, 46, 115, 113, 108, 46, 75, 114, 121, 111, 67, 108, 97, 115, 115, 68, 97, 116, -31, 1, 1, -126, 97, 2]|
|[1, 0, 111, 114, 103, 46, 97, 112, 97, 99, 104, 101, 46, 115, 112, 97, 114, 107, 46, 115, 113, 108, 46, 75, 114, 121, 111, 67, 108, 97, 115, 115, 68, 97, 116, -31, 1, 1, -126, 98, 4]|
|[1, 0, 111, 114, 103, 46, 97, 112, 97, 99, 104, 101, 46, 115, 112, 97, 114, 107, 46, 115, 113, 108, 46, 75, 114, 121, 111, 67, 108, 97, 115, 115, 68, 97, 116, -31, 1, 1, -126, 99, 6]|
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
```
After the fix, it will be like the below if and only if the users override the `toString` function in the class `KryoClassData`
```scala
override def toString: String = s"KryoClassData($a, $b)"
```
```
+-------------------+
|value              |
+-------------------+
|KryoClassData(a, 1)|
|KryoClassData(b, 2)|
|KryoClassData(c, 3)|
+-------------------+
```

If users do not override the `toString` function, the results will be like
```
+---------------------------------------+
|value                                  |
+---------------------------------------+
|org.apache.spark.sql.KryoClassData68ef|
|org.apache.spark.sql.KryoClassData6915|
|org.apache.spark.sql.KryoClassData693b|
+---------------------------------------+
```

Question: Should we add another optional parameter in the function `show`? It will decide if the function `show` will display the hex values or the object values?

Author: gatorsmile <gatorsmile@gmail.com>

Closes #10215 from gatorsmile/showDecodedValue.