accessor function for key-field

### Clear and concise description of the problem

We have data which for few reasons has all of its field in arrays like this:
```json
[
{
"id": ["12345"],
"users": ["Anton", "John"]
},
....
]
```
Currently I didn't find a solution of how to make the `key-field` see the id.

### Suggested solution

It would be great if the `key-field` would accept the accessor function, telling how to get the `key` from a row of data, like this:
```vue

```

### Alternative

_No response_

### Additional context

_No response_

### Validations

- [X] Read the [docs](https://github.com/Akryum/vue-virtual-scroller/blob/master/packages/vue-virtual-scroller/README.md).
- [X] Check that there isn't [already an issue](https://github.com/Akryum/vue-virtual-scroller/issues) that reports the same bug to avoid creating a duplicate.