Support `singularity://`or `file://` protocol in WDL `container` field to allow using SIF files directly

[adamnovak]

See MiniWDL issue chanzuckerberg/miniwdl#792

We would like to be able to run a WDL that has container: "singularity:///absolute/path/to/sif/file.sif".

We probably should also allow singularity://relative/path/to/sif/file.sif.

┆Issue is synchronized with this Jira Story
┆Issue Number: TOIL-1800