[SPARK-3412][SQL]add missing row api

@chenghao-intel assigned this to me, check PR #2284 for previous discussion
You don't want the first `.asInstanceOf`here as it can cause errors.

```scala
scala> new SpecificMutableRow(IntegerType :: Nil).getAs[Int](0)
java.lang.ClassCastException: org.apache.spark.sql.catalyst.expressions.MutableInt cannot be cast to org.apache.spark.sql.catalyst.expressions.MutableAny
```

Since all `MutableValue` classes have a `.value` method I think you can just leave it out.