Support multi-arch Singularity

[unito-bot]

Toil should support some way to do its Singularity image caching for CWL (and WDL?) that can handle multiple architectures sharing a cache.

Can a SIF be multi-arch like a Docker container? Do we have to make arch-specific cache directories under the user-provided path? Can we get the CWL pre-puller to pre-pull all the relvant architectures?

We might need to coordinate with cwltool on this, since I think it actually reads the cache on the CWL side, and kind of owns the pre-puller.

┆Issue is synchronized with this Jira Story
┆Issue Number: TOIL-1688