[Variant] feat: add support for casting MapArray to VariantArray

[Weijun-H]

Which issue does this PR close?

• Closes [Variant]: Implement DataType::Map support for cast_to_variant kernel #8063

Rationale for this change

Maps are now cast to Variant::Objects

What changes are included in this PR?

Are these changes tested?

Yes

Are there any user-facing changes?

[alamb]

Thank you @Weijun-H -- I have some questions

[alamb]

This seems to have created something like

[{ 
  "keys": "key1",
  "values": 1
}]

I expect the result to be

{ "key1": 1 }

Questions:

1. where did the list come from?
2. I am not sure about the "keys" and "values" fields 🤔

[Weijun-H]

Since there is no data type in Variant that can directly represent a map,
I converted the map into the following structure:

[
{ 
  "keys": "key1",
  "values": 1
},
{ 
  "keys": "key2",
  "values": 2
}
]

Using an object might be a better idea, but the keys can only be strings. 🤔

[alamb]

I think object is the more expected result

Maybe you can all the keys of the MapArray into strings using the cast_to_variant kernel ?

[alamb]

Thanks @Weijun-H -- this is looking good

[alamb]

We could also potentially cast this directly to a string aray here, somethin glike

Suggested change

	let keys = cast_to_variant(map_array.keys())?;
	let keys = cast(map_array.keys(), DataType::Utf8)?;

And then you wouldn't have to worry about dealing with Variants

I think what you have here, other than the unwrap, is looks good to me

[alamb]

Thank you @Weijun-H

[alamb]

I merged up to resolve a conflict

[alamb]

Thanks. again @Weijun-H