`FILTER` clause in `SELECT` clause

In SQL we can add a `FILTER` clause to an aggregate `SELECT` clause, i.e.

```sql
SELECT [item], SUM([price]) AS [total], SUM([price]) FILTER ( WHERE [date] >= '2021-01-01' ) AS [sales2020]
FROM [orders]
GROUP BY [item]
```

It would be a huge help if SqlKata can support passing `WHERE` clauses with their bindings to a `SELECT` clause.