API support for content-hash-based versioning

[theoy]

Hash-based versioning is a transform, but it always need to be applied last - with a sentinel value (could be empty or 0) in all the locations that the hash will go, and then replaced at the very end.

To support this in a less error-prone way, ideally it would be special-cased as a transform.

Also, ideally it would be easy to opt in (since this is a good way to version your pre-releases).