Add support for StructArray to `ScalarValue`

[jorgecarleitao]

Enabling DataFusion to support StructArray for some of its scalar operations.

Note that this may be difficult to support in Ballista due to how scalar values are currently represented in the proto, so I suggest we restrict this to DataFusion for now.

[jonmmease]

I would like to work on this. I have a local branch where I've added the following as a new ScalarValue variant:

Struct(Option<Box<Vec<ScalarValue>>>, Box<Vec<Field>>)

mirroring the current List variant

List(Option<Box<Vec<ScalarValue>>>, Box<DataType>)

Does that make sense as a starting point?

[houqp]

sorry for the delay @jonmmease , i think so, please feel free to send a PR and we can discuss further there.