Allow querying of multiple files at once

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As a developer of an application that uses the Validator API,
I want to query for the validation results for multiple files at once
so that I can provide information to my users in a timely fashion .

In IATI/ckanext-iati#417 we're discussing using the Validator API to power a summary view of publishers.

In IATI/js-validator-api#543 I discuss response speed, which would provide a partial solution. I would expect this functionality to be of a similar speed to querying for a single file.

An alternative would be to allow the user to request reports for multiple files at once: in the use case we're discussing this would be in the order of tens to hundreds of reports. Alongside IATI/js-validator-api#542 this would make for a powerful apparatus for querying summary data.