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/Directory 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.