Avoid re-evaluating expressions in filters and projections

This patterns shows up pretty often:

```sql
select expensive(col)
from t
where expensive(col)
```

A pathological case is variant / json:

```sql
select variant_get(col, 'key')
from t
from variant_get(col, 'key')
```

There's two issues here:
1. Until we solve projection pushdown (https://github.com/apache/datafusion/issues/14993) if `key` is not shredded we materialize the entire `col` and then extract `key` in a `ProjectionExec`.
2. Even once that is resolved, or in the case that `key` is not shredded evaluating `variant_get(col, 'key')` itself is expensive we still re-compute `variant_get(col, 'key')` twice: once for the filter and once for the projection.

Start by reading the filter and projection behavior described in the SQL examples, including ProjectionExec and the projection-pushdown issue #14993. Determine how repeated expressions such as variant_get(col, 'key') are represented and evaluated, then define completion as avoiding duplicate evaluation while preserving the query results.

Written by the indexing model from the issue text.