Add protobuf serialisation for Arrow Map types

[ahmedriza]

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

Currently, the protobuf serialisation does not support Arrow Map types. This means that any query from ballista on, say, a Parquet file that contains Map data structures will fail to serialise the Arrow Schema, and hence lead to failure of the whole query (regardless of whether the query actually touches the Maps in the Parquet).

Describe the solution you'd like

Add the missing functionality to serialise Arrow Map data types to datafusion/proto/proto/datafusion.proto and the related serialisers.

Describe alternatives you've considered

I could not see any alternatives.

Additional context
Using this parquet file:
map.parquet.gz

and the following code:

async fn ballista_query() {
    let config = BallistaConfig::new().unwrap();
    let ctx = BallistaContext::standalone(&config, 10).await.unwrap();
    ctx.register_parquet("t", "map.parquet", ParquetReadOptions::default()).await.unwrap();
    let df = ctx.sql("select * from t").await.unwrap();
    df.show().await.unwrap();
}

will fail with:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Internal("failed to serialize logical plan: Plan(\"General error: Proto serialization error: The Map data type is not yet supported\")")',

A correct execution should produce:

+------+------------------------------+
| name | properties                   |
+------+------------------------------+
| foo  | {symbol: FOO, currency: GBP} |
| bar  | {symbol: BAR, currency: USD} |
| baz  | {symbol: BAZ, currency: EUR} |
+------+------------------------------+