Can vue2 key-field support multiple

### Clear and concise description of the problem

Can vue2 key-field support multiple values and accept a function like vue3

### Suggested solution

Can vue2 key-field support multiple values and accept a function like vue3

### 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.