Use original value when comparing with dictionary column in unparser

[Sevenannn]

Which issue does this PR close?

No relevant issue

Rationale for this change

Datafusion enforces an eager cast of value to dictionary value in the logical plan where a comparison with dictionary value is presented. For example, in the following plan

Filter: person_mood.mood_status = CAST(Utf8("happy") AS Dictionary(Int8, Utf8)) 

However, when using unparser to convert the plan back to sql, where the sql will be sent to various query engine (PostgreSQL, MySQL, etc.) the cast is not needed for the value, in the example above the value happy, since the raw value can be directly used in SQL engines without casting to an engine specific dictionary type. Therefore, the plan can simply be rewritten into

where person_mood.mood_status = 'happy' 

What changes are included in this PR?

• Directly pass inner_expr instead of cast inner_expr as dictionary in the Expr::Cast case when the inner_expr is a Value representing constant and the plan is cast to dictionary
• Add a test for the change

Are these changes tested?

Yes

Are there any user-facing changes?

No

[phillipleblanc]

I think this change makes sense, both from a readability of the generated SQL point of view, and also for making it work to send this SQL to remote query engines.

I do wonder if there is some edge case that we aren't thinking of where we would need to preserve this cast? 🤔 If so, then I think at that time we would make this a Dialect option and unparse it different based on the dialect. For now I think this is fine!

Thanks @Sevenannn!

[alamb]

Thank you @Sevenannn and @phillipleblanc

[alamb]

this is another example of where logical types as discussed in #11513 might make sense