Support partially-embargoed Dandisets

This was spun off of #8, due to being blocked on implementation of https://github.com/dandi/dandi-archive/pull/1772.

Once the feature is implemented in the Archive, `backups2datalad` will need to support partially-embargoed Dandisets (i.e., Dandisets in which some but not all assets are embargoed).

Questions that need answering (including re-checking the answers after the Archive implementation):

* What is the `embargo_status` of a partially-embargoed Dandiset?
* Answer per https://github.com/dandi/backups2datalad/issues/8#issuecomment-1860951062: either `OPEN` or `EMBARGOED`

* How can a consumer of the API determine whether an individual asset is embargoed or not?
* Answer per https://github.com/dandi/backups2datalad/issues/8#issuecomment-1860951062: By checking whether the `"access"` field of the metadata is `"dandi:OpenAccess"` or `"dandi:EmbargoedAccess"`.

* When backing up open assets in an embargoed Dandiset, how (if at all) should the backups differ from backups of embargoed assets in the Dandiset?

* When backing up a blob asset in an embargoed Dandiset where the blob is also used by open assets in open Dandisets, what special consideration should be given to the asset?
* Q: How can such blobs/assets be identified?
* Yarik's comments in #8 indicate that he wants such assets to have their S3 URLs recorded.

* When backing up embargoed assets in an open Dandiset, how (if at all) should the backups differ from backups of open assets in the Dandiset?
* Should such assets have their metadata stored in `.dandi/assets.json`?
* Should such assets have their S3 URLs registered?