`lag` and `lead` function support

I want to do a select as the following:

```sql
SELECT
LAG(a.id, 1) OVER (
ORDER BY a.sent_at DESC
) previous_a,
lead(a.id, 1) OVER (
ORDER BY a.sent_at DESC
) next_a
FROM
...
```

Since `esqueleto` doesn't have lag and lead yet, I want to create it using raw sql, but I believe using `unsafeSqlFuntion` doesn't work due to having `over` clause.

Hope someone could point me to the right direction.

Start by examining the existing unsafeSqlFuntion entry point and how Esqueleto represents SQL expressions and OVER clauses. Determine the API needed to express LAG and LEAD with ordering, then verify that the example query can be represented and tested; the issue provides no file or test path to begin from.

Written by the indexing model from the issue text.