ESQL: Support "Look around" "Buckets"

### Description

There are use-cases with data where it would be nice to be able to get either the previous or next row in a query to do additional analysis (sparse data sets)

Example:

If I have data like (**Note:** The `@timestamp` values):

| `@timestamp` | `index` | `num_of_docs` |
| -------------- | ------ | --------------- |
| 2024-11-04T00:00:00.000Z | index_1 | 0 |
| 2024-11-04T00:00:**1**0.000Z | index_1 | 1 |
| 2024-11-04T00:**1**0:00.000Z | index_1 | 3 |
| 2024-11-04T0**1**:00:00.000Z | index_1 | 6 |
| 2024-11-04T**1**0:00:00.000Z | index_1 | 12 |

Now, I would like to compute the rate (difference over time) for the `num_of_docs` in `index` over `@timestamp`.

However, I want to keep my bucket size at ~5 minutes.

Ideally, I'd like an output like:

| `@timestamp` | `index` | `num_of_docs` | `num_of_docs_diff` |
| --------------- | ------- | --------------- | ---------------------- |
| 2024-11-04T00:00:00.000Z | index_1 | 0 | null/0 |
| 2024-11-04T00:05:00.000Z | index_1 | 1 | 1 |
| 2024-11-04T00:**1**0:00.000Z | index_1 | 3 | 2 |
| 2024-11-04T00:**1**5:00.000Z | index_1 | 3 | null/0 |
| ... | ... | ... | ... |
| 2024-11-04T00:55:00.000Z | index_1 | 3 | null/0 |
| 2024-11-04T0**1**:00:00.000Z | index_1 | 6 | 3 |
| 2024-11-04T01:05:00.000Z | index_1 | 6 | null/0 |
| ... | ... | ... | ... |
| 2024-11-04T**0**9:55:00.000Z | index_1 | 6 | null/0 |
| 2024-11-04T**1**0:00:00.000Z | index_1 | 12 | 6 |
| 2024-11-04T**1**0:05:00.000Z | index_1 | 12 | null/0 |

My ES|QL query today would look something like:

```
FROM metrics-*
| STATS max = MAX(`num_of_docs`), min = MIN(`num_of_docs`)
BY index, BUCKET(@timestamp, 5 minute)
| EVAL num_of_docs_diff = max - min
```

This won't work in our case, as the bucket interval is too small to be able to reliably capture the previous event in the data set as it is outside of the bucket window.

Being able to have a way to tell ES|QL to have two different "BUCKETS" would be useful here, where one of the "BUCKETS" would be the "normal" 5 minute one for actual grouping, but then having a second "BUCKET" be somewhat like a "look around" bucket, where a STATS function would check outside the first BUCKETs range for additional values.

(I'm sure there are better words to use for this type of feature, so feel free to update/change)

No source file or test is named in the issue. Start by reading the ESQL BUCKET and STATS query behavior described here, then determine how the requested look-around bucket should interact with five-minute grouping. Done means a settled design and implementation that produces the shown previous-value differences without losing empty buckets.

Written by the indexing model from the issue text.