Support arrow map datatype

[nevi-me]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

There is an arrow + parquet user that has encountered the need for a arrow::datatypes::DataType::Map. A map is a logical type for a physical type List<Struct<key={Primitive},value={Primitive}>>, so it shouldn't be as hard to implement as say Union.

Describe the solution you'd like

To add support for DataType::Map in the least possible way required to unblock the user. i.e. we might not want to support it in arrow::compute, but might want to support it in:

• arrow::array
• arrow::datatypes
• arrow::json
• arrow::ipc

Describe alternatives you've considered

None considered.

Additional context

Here's a thread describing the issue (not sure if it requires signing in) https://delta-users.slack.com/archives/C013LCAEB98/p1622663872101400

[nevi-me]

Fixed by #491