Add ability to request additional OPTIMADE fields

[ml-evs]

Summary

Closes #2314 by adding the additional_response_fields argument when generating SNLs from OPTIMADE requests.

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[coveralls]

Coverage decreased (-0.6%) to 83.127% when pulling 642431e on ml-evs:ml-evs/optimade_response_fields into dd37370 on materialsproject:master.

[mkhorton]

Thanks @ml-evs, this is useful, I appreciate you have this working with the SNL objects too.