OptimadeRester: Add ability to request additional response fields

[ml-evs]

We have just run into an issue (Materials-Consortia/optimade-tutorial-exercises#8) where the pymatgen OPTIMADE tutorial fails to pull custom provider fields since #2244 was merged, as this PR (correctly) enforced the response fields required to create pymatgen structures from OPTIMADE responses, to the exclusion of all other fields.

As a quick fix, I suggest adding a additional_response_fields argument to the public methods of OptimadeRester that are used when constructing the query.

I am happy to knock up a PR if this is desired, but for now we will probably have to remove that section of the pymatgen tutorial.

(paging @mkhorton - the tutorials are now being used for a new workshop, hence the interest)