Add a filegetter for CSV toolchains that fetches a file from a URL

[mjordan]

If the CSV input file has a field that contains a URL to a PDF, image, etc., we should be able to write a FileGetter that fetches the specified file.

[mjordan]

Probably should only apply (at first anyway) to CsvSingleFile toolchain.

[mjordan]

Looking at this, the filegetter just gets the path to the file, not the contents of the file. That happens in writer via a copy() operation. This path is also validated by the inputValidator. OK, so this is not as straight forward as it seemed.