Add dictionary support to unhex and test dictionary and scalar cases

[kazuyukitanimura]

Describe the bug

Right now unhex does not handle dictionary types. (Scalar is supported but we do not have tests)
Need to add tests for both dictionary and scalar types

Steps to reproduce

No response

Expected behavior

No response

Additional context

No response

[kination]

@kazuyukitanimura sorry for question.
Is it meaning unhex cannot decode correctly if value is dict? For example...

SELECT decode(unhex('<source>'), 'UTF-8'); 

it works well if source hex data is String such as Spark, but it fails if it is { "target": "spark" }
-> Am I understanding correct?

[kazuyukitanimura]

Thanks @kination
In the unhex test https://github.com/apache/datafusion-comet/blob/main/spark/src/test/scala/org/apache/comet/CometExpressionSuite.scala#L1433
We only have limited test cases.
You can use makeParquetFileAllPrimitiveTypes to create test dictionary values and some random values

The test case with dictionary may fail because I do not see dictionary handling at https://github.com/apache/datafusion-comet/blob/main/native/spark-expr/src/math_funcs/unhex.rs#L67

It is possible the tests with dictionary still passes if we are unpacking in advance. But I forgot we are doing so