Reorganize classes/re-architecture code

*WIP design document for a new code architecture*

* Add a `DandisetBackup` class that takes care of all repository-wide operations for backing up a Dandiset
* Primary attributes:
* `dandiset: RemoteDandiset`
* `ds: AsyncDataset`
* `manager: Manager`
* Merge all `Syncer` methods into a single method of this class
* Move the following `Datasetter` methods to this class:
* `update_dandiset()`
* `ensure_github_remote()`
* `sync_dandiset()`
* part of `update_github_metadata()`?
* `tag_releases()`
* `mkrelease()`
* `backup_zarrs()`
* Make `update_dandiset_metadata()` a method of this class
* Make `async_assets()` a method of this class
* Make `register_s3urls()` a method of this class

* Add a `ZarrBackup` class for subsuming `ZarrSyncer`?

* Wrap [this block](https://github.com/dandi/backups2datalad/blob/53478a80b89a712cb5d3dd59224938b5aa844aa9/src/backups2datalad/asyncer.py#L518-L532) in a function that takes a `DandisetBackup`, `aiterassets()` async iterator, `tracker`, & `error_on_change` and returns a structure containing `Report`, `dm.last_timestamp`, `dm.need_add`, & `dm.zarrs`

* Move the `addurl` methods & attributes of `Downloader` to an `Addurl`(?) class, an instance of which will be passed to `Downloader` on instantiation

* Add a `Superdataset` class that takes care of operations on the superdataset