schema: version and its annotation

@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
```python
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: https://github.com/dandi/dandi-api/issues/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?)