Does the `url` field in the TES output require a scheme?

All of the examples in the specification require a scheme to be present (`gs://`, `s3://`, `file://`), but the specification does not explicitly state that a scheme is required. I had an example today of a TES server that wanted a path with no scheme to upload to object storage. So my questions are,

- Are schemes required to be present in the `url` field, and
- Is there a defined list of supported schemes or is that implementation specific?

Learning from WDL, it might be beneficial to require that at least a handful of schemes are supported for cross-platform compatibility.