Feature request: rolling window

### Describe the feature
Introduce a `rolling_window` macro that allows users to calculate trailing-window aggregates (e.g., 7-day rolling sums) by duplicating fact table rows into the preceding N days and then grouping.

### Describe alternatives you've considered

The problem with `WINDOW` functions is that the table may be missing rows for certain days, then some data will be also missing from the output (even though it shouldn't).

Example: Daily Rainfall Averages

We have a `rainfall_daily` with the average rainfall per city per day, but only when it rained.
city | day_dt | mm_rain
-------|------------|---------
Oslo | 2023-08-25 | 4.2
Oslo | 2023-08-27 | 3.5
Oslo | 2023-08-30 | 6.1

Now our task is to calculate the 7-day rolling rainfall average. If we use a window function (`SUM(mm_rain) OVER (PARTITION BY city ORDER BY day_dt RANGE BETWEEN INTERVAL '6 days' PRECEDING AND CURRENT ROW)`),
we only get rows for the dates in the source table (`25, 27, 30`). But we actually want results for every date (25 → 31), so that a dashboard chart shows a continuous line. On August 31, for example, Oslo’s rainfall average should include rain on the 25th, 27th, and 30th (even though Aug 31 has no row in the table).

city | end_date | start_date | sum_mm_rain | avg_mm_rain
-------|------------|------------|-------------|------------
Oslo | 2023-08-25 | 2023-08-19 | 4.2 | 0.6
Oslo | 2023-08-26 | 2023-08-20 | 4.2 | 0.6
Oslo | 2023-08-27 | 2023-08-21 | 7.7 | 1.1
Oslo | 2023-08-28 | 2023-08-22 | 7.7 | 1.1
Oslo | 2023-08-29 | 2023-08-23 | 7.7 | 1.1
Oslo | 2023-08-30 | 2023-08-24 | 13.8 | 2.0
Oslo | 2023-08-31 | 2023-08-25 | 13.8 | 2.0

### Additional context

To my knowledge, this isn't database-specific.

### Who will this benefit?

Any place where a rolling window is needed, but the source table is sparse (missing records for certain combination of unique keys).

### Are you interested in contributing this feature?
Yes