Support embargoed Dandisets

[jwodder]

Closes #8.

To do:

• [x] Decide how to handle assets missing SHA256 digests when registering S3 URLs (https://github.com/dandi/backups2datalad/issues/8#issuecomment-1874502017)
• [x] When a Dandiset is unembargoed, make the GitHub repository public
• [x] Add a command for just registering S3 URLs
• [x] Add tests

[codecov[bot]]

Codecov Report

Attention: 42 lines in your changes are missing coverage. Please review.

Comparison is base (6a3b037) 75.74% compared to head (af3934f) 75.70%. Report is 1 commits behind head on main.

Files	Patch %	Lines
src/backups2datalad/__main__.py	47.05%	8 Missing and 1 partial :warning:
src/backups2datalad/register_s3.py	73.52%	6 Missing and 3 partials :warning:
src/backups2datalad/asyncer.py	83.33%	4 Missing and 4 partials :warning:
src/backups2datalad/adataset.py	77.27%	4 Missing and 1 partial :warning:
src/backups2datalad/syncer.py	83.33%	3 Missing and 2 partials :warning:
src/backups2datalad/adandi.py	70.00%	2 Missing and 1 partial :warning:
src/backups2datalad/datasetter.py	62.50%	2 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #12 +/- ## ========================================== - Coverage 75.74% 75.70% -0.04% ========================================== Files 15 17 +2 Lines 2544 2680 +136 Branches 562 589 +27 ========================================== + Hits 1927 2029 +102 - Misses 458 482 +24 - Partials 159 169 +10 ```

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[jwodder]

@yarikoptic Ping.