schema: version and its annotation

[yarikoptic]

@satra caught me in the middle of writing this, so I will just submit what I had, but intermediate agreement was wasGeneratedBy will be the remedy (details yet to be worked out ;))

ATM we have

class DandisetMeta(CommonModel, Identifiable):
    ...
    version: TempOptional[str] = Field(readOnly=True, nskey="schema")

dandi-api returns also created/modified time stamps (not part of the metadata but just come from DB), and I already requested addition of "description" to publish call: dandi/dandi-archive#189 . Also the notion of a Version is present within perspective Python client/API redesign: https://github.com/dandi/dandi-cli/pull/447/files#diff-ca726fb8fbfbf61284ae89742c7c5621e5d4305fc9fd8a83047f71eb07a28379R89 which ATM mimics it more from dandi-api returned structure.

I feel that if we are to have version within metadata, we should also encapsulate other related to that publishing process metadata (date of publishing, and optionally description). I wonder where should it "fit" in the metadata record for a published dandiset? I guess it could be added to wasGeneratedBy.

But ATM we override it to be of type Optional[List[Project]] from the original (in CommonModel) "wider" Optional[List[Activity]]. Should we then make it Optional[List[Union[Project,Activity]] (although Project is subclass of Activity, I guess it would be useful to direct web UI?)