[FT] Build in a way to specify specific IDs/Lines in Dataset to use as few-shot examples in the same split

## Solution/Feature
When creating a custom task, the few-shot samples can be specified via a split from the dataset. However, it would be convenient if there were a way to make it so I can specify samples from the split that will be used as the examples, similar to what LM Evaluation Harness does here in its yaml config:

```
test_split: train
fewshot_split: train
fewshot_config:
sampler: id_sampler
sampler_config:
id_list: ["2022_21", "2022_88", "2022_143"]
id_column: id
exclude_from_task: true

```

Something along the lines of:
```
@dataclass
class LightevalTaskConfig:
"""Stored configuration of a given [`LightevalTask`].

Arguments:
...
hf_subset (str): Subset used for the current task, will be default if none is selected.
hf_avail_splits (list[str]): All the available splits in the evaluation dataset
evaluation_splits (list[str]): List of the splits actually used for this evaluation
few_shots_split (str): Name of the split from which to sample few-shot examples
few_shots_id_column (str): Column name in the dataset to use for the few_shot_id_list
few_shots_id_list (list[str]): List of unique identifiers corresponding to the samples in the dataset, based on matching these IDs with the values in few_shots_id_column.
few_shots_select (str): Method with which to sample few-shot examples
...
"""
```

The new arguments would be `few_shots_id_column` and `few_shots_id_list`, and wrote a rough description of what they would look like.