Allow version notes in "datasets create" or allow "datasets version" to create a dataset if it doesn't exist yet

Hi,

Would you please be able to add these two features?
(if they don't require API change on the backend, I can implement them myself and submit a PR)

There are two issues:
- Creating a new dataset CLI API doesn't allow to set version notes (thus initial version always says "Initial release")
- When I use this in a notebook, I need to put checks that if it's the first time the dataset is being uploaded, then do `datasets create`, else do `datasets version`, when ultimately these are the same things. It would be much more convenient if `datasets version` would create a new dataset if one doesn't exist yet, thus there would be need only for one command without any checks.