Add unembargo of Zarrs and extend test_backup_embargoed with a zarr

[yarikoptic]

Came up on slack while discussing @jjnesbitt merge of Zarrbargo

• Add API support for Embargoed Zarrs dandi-archive#2069

so we need to now add handling of unembargoing of Zarrs into utility here and extend the test (ref: https://github.com/dandi/backups2datalad/blob/HEAD/test/test_commands.py#L367) to include a zarr.

Might be of interest to @aaronkanzer