[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.