COUNT DISTINCT does not support dictionary types

[alamb]

As in #199

Describe the bug
Distinct aggregates such as select count(distinct col) ... error when col is a dictionary

To Reproduce

            let mut ctx = ExecutionContext::new();

            // input data looks like:
            // A, 1
            // B, 2
            // A, 2
            // A, 4
            // C, 1
            // A, 1

            let dict_array: DictionaryArray<K> =
                vec!["A", "B", "A", "A", "C", "A"].into_iter().collect();
            let dict_array = Arc::new(dict_array);

            let val_array: Int64Array = vec![1, 2, 2, 4, 1, 1].into();
            let val_array = Arc::new(val_array);

            let schema = Arc::new(Schema::new(vec![
                Field::new("dict", dict_array.data_type().clone(), false),
                Field::new("val", val_array.data_type().clone(), false),
            ]));

            let batch = RecordBatch::try_new(schema.clone(), vec![dict_array, val_array])
                .unwrap();

            let provider = MemTable::try_new(schema.clone(), vec![vec![batch]]).unwrap();
            ctx.register_table("t", Arc::new(provider)).unwrap();


            // Now, use dict as an aggregate
            let results =
                plan_and_collect(&mut ctx, "SELECT val, count(distinct dict) FROM t GROUP BY val")
                    .await
                    .expect("ran plan correctly");

            let expected = vec![
                "+-----+-------------+",
                "| val | COUNT(dict) |",
                "+-----+-------------+",
                "| 1   | 2           |",
                "| 2   | 2           |",
                "| 4   | 1           |",
                "+-----+-------------+",
            ];
            assert_batches_sorted_eq!(expected, &results);

Fail with an error such as:

thread 'execution::context::tests::group_by_dictionary' panicked at 'ran plan correctly: ArrowError(ExternalError(NotImplemented("Can\'t create a scalar of array of type \"Dictionary(Int8, Utf8)\"")))', datafusion/src/execution/context.rs:1774:22
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Expected behavior
test passes

Additional context
Add any other context about the problem here.