Insert Extractive QA templates for SQuAD-like datasets
This PR adds task templates for 9 SQuAD-like templates with the following properties:

* 1 config
* A schema that matches the `squad` one (i.e. same column names, especially for the nested `answers` column because the current implementation does not support casting with mismatched columns. see #2434)
* Less than 20GB (my laptop can't handle more right now)

The aim of this PR is to provide a few datasets to experiment with the task template integration in other libraries / services. 

PR #2429 should be merged before this one.

cc @abhi1thakur 