feature: allow URLs for File inputs read in from an inputs file

[peterhuene]

With the initial implementation of task evaluation in #265, input files can be co-located with the JSON inputs file, if desired.

However, it is very useful to accept URLs as File inputs in the JSON inputs file.

The wdl run tooling should download the file (and perhaps provide a configurable download cache) and feed the download path through as the File value.