Optimize CASE expression to produce dictionary-encoded arrays in some cases

### Is your feature request related to a problem or challenge?

Here is an example `CASE` expression that produces an array containing exactly two distinct values:

```sql
CASE WHEN expr THEN 1 ELSE 0 END
```

We currently produce an array containing ones and zeros (and nulls). It seems like it would be beneficial to produce a dictionary-encoded array in this case to reduce memory pressure. This could also make subsequent operations more efficient in some cases.

No implementation file, test, or entry point is identified in the issue. Start by locating CASE expression execution and existing dictionary-encoding support, then verify that the example produces a dictionary-encoded array while preserving ones, zeros, and nulls and improving subsequent operations where applicable.

Written by the indexing model from the issue text.