Various objects stores provide etags in their FileStatus implementations
Make these values accessible
- new interface EtagFromFileStatus to be implemented when provided
- filesystem.md to declare requirements of etags (constant between LIST and HEAD)...
- path capabilities for (a) etag and (b) etags consistent across rename
Add implementation for abfs, later s3a (and google gcs)
This is initially to handle recovery from certain failures in job commit against abfs, but it would allow a cloud-ready version of distcp to track etags of uploaded files, so diff properly.