Protobuf serialisation is missing for GetIndexedFieldExpr

[ahmedriza]

Describe the bug
The protobuf serialisation is missing for GetIndexedFieldExpr which causes ballista to fail to execute SQL queries that index fields of nested types.

To Reproduce

Use the attached parquet file:

and run the following code

#[tokio::test]
async fn test_ballista_sql() {
    let config = BallistaConfig::builder().build().unwrap();
    let ctx = BallistaContext::standalone(&config, 10).await.unwrap();
    ctx.register_parquet("t", "list.parquet", ParquetReadOptions::default()).await.unwrap();
    let df = ctx.sql("select id, a[1], a[2], a[3] from t").await.unwrap();
    df.show().await.unwrap();
}

This will fail with

Error: Arrow error: External error: Execution error: Job F5d0DAb failed: Error planning job F5d0DAb: DataFusionError(Internal("physical_plan::to_proto() unsupported expression GetIndexedFieldExpr { arg: Column { name: \"a\", index: 0 }, key: Int64(1) }"))

Expected behavior

Should produce the following output:

+----+--------+--------+--------+
| id | t.a[1] | t.a[2] | t.a[3] |
+----+--------+--------+--------+
| 1  | 1.71   | 2.71   | 3.71   |
+----+--------+--------+--------+

Additional context
This is due to the missing protobuf serialisations for GetIndexedFieldExpr