Refactor column mappings for question answering datasets
This PR tweaks the keys in the metadata that are used to define the column mapping for question answering datasets. This is needed in order to faithfully reconstruct column names like `answers.text` and `answers.answer_start` from the keys in AutoTrain.

As observed in https://github.com/huggingface/datasets/pull/4367 we cannot use periods `.` in the keys of the YAML tags, so a decision was made to use a flat mapping with underscores. For QA datasets, however, it's handy to be able to reconstruct the nesting -- hence this PR.

cc @sashavor 