Support filename coercion back to string

[stxue1]

The spec expects that filenames can be represented as a string to alter things like filename extensions: https://github.com/openwdl/wdl/blob/9c0b9cf4586508a9e6260cc5c5e562e21f625aac/SPEC.md?plain=1#L6521

We virtualize files into our own representation, so coercing back will give us a toilfile URI instead of the filename:

data_file = "toilfile:4%3A0%3Afiles%2Ffor-job%2Fkind-WDLTaskJob%2Finstance-kfz8ammc%2Ffile-c91abfb5298d4a83bad4bb00d53beb55%2Ffoo.data/e946ed9a-784f-47d3-bf74-e84a23d9056d/foo.data"

┆Issue is synchronized with this Jira Story ┆Issue Number: TOIL-1609