Add an option to mount WDL inputs as a fully realized directory of files to allow unlimited input files to Singularity containers

[adamnovak]

When we run a WDL task with Singularity, we mount each input file into the container individually, and each mount generates a command-line argument to singularity.

The total length of command line arguments is limited, and the net result is that once you get over something like 10,000 input files it breaks.

We should have an option/mode/fallback where we just copy out/hardlink all the necessary input files into a directory with the structure we want to see inside the container, and then mount just the one directory in.

This could maybe ride some of the input directory synthesis rails we will need for #4848 and #4815 to show you the files as laid out inside the container.

┆Issue is synchronized with this Jira Story
┆Issue Number: TOIL-1534