[SPARK-21445] Make IntWrapper and LongWrapper in UTF8String Serializable

## What changes were proposed in this pull request?

Making those two classes will avoid Serialization issues like below:
```
Caused by: java.io.NotSerializableException: org.apache.spark.unsafe.types.UTF8String$IntWrapper
Serialization stack:
    - object not serializable (class: org.apache.spark.unsafe.types.UTF8String$IntWrapper, value: org.apache.spark.unsafe.types.UTF8String$IntWrapper326450e)
    - field (class: org.apache.spark.sql.catalyst.expressions.Cast$$anonfun$castToInt$1, name: result$2, type: class org.apache.spark.unsafe.types.UTF8String$IntWrapper)
    - object (class org.apache.spark.sql.catalyst.expressions.Cast$$anonfun$castToInt$1, <function1>)
```

## How was this patch tested?

- [x] Manual testing
- [ ] Unit test

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #18660 from brkyvz/serializableutf8.

common/unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java

@@ -863,8 +863,8 @@ public final class UTF8String implements Comparable<UTF8String>, Externalizable,
    * Wrapper over `long` to allow result of parsing long from string to be accessed via reference.
    * This is done solely for better performance and is not expected to be used by end users.
    */
-  public static class LongWrapper {
-    public long value = 0;
+  public static class LongWrapper implements Serializable {
+    public transient long value = 0;
   }
 
   /**
@@ -874,8 +874,8 @@ public final class UTF8String implements Comparable<UTF8String>, Externalizable,
    * {@link LongWrapper} could have been used here but using `int` directly save the extra cost of
    * conversion from `long` to `int`
    */
-  public static class IntWrapper {
-    public int value = 0;
+  public static class IntWrapper implements Serializable {
+    public transient int value = 0;
   }
 
   /**