Supply a required property for each inputField

[theosanderson]

inputFields are the fields used to define what input names the preprocessing pipeline is expecting. While these often correspond to metadata field names technically they can and will sometimes be distinct.

Some inputFields are required, and the schema should capture this to allow the website to display things accordingly.

(Cornelius refactored inputFields to be "DRY" and largely derived from the main field spec, for these we may be able to pass on the required property from the main field - but we should think about if that is actually the same property. My own instinct is to be less DRY and just to set out the inputFields explicitly separately, but this may not be required.)

[fhennig]

My intuition when reasoning about these fields would be that any overlap with the metadata fields is incidental and that they are semantically distinct. I can see how it might be nice to have some tooling or syntactical support to allow for brevity when configuring them though.