Add function to update a CAR file from a CAR slice (diff).

[groceryheist]

I want to periodically update a local snapshot (repo_snapshot) of a repo with a known did repodid. The snapshot is a CAR_file corresponding to a known commit rev. I can get a CAR file which is a "diff" (aka CAR slice) between the snapshot and the current repo state:

paramsdict = models.ComAtprotoSyncGetRepo.ParamsDict(did=repodid, since=rev)
repo_diff = client.com.atproto.sync.get_repo(params=paramsdict)

I want to update my snapshot CAR file from the diff. I imagine a function such as

updated_snapshot = CAR.utils.update(repo_snapshot, repo_diff)

The reason is to efficiently keep an almost-up to date local copy of a repo using bandwidth efficiently without having to handle the firehose.

This functionality might already be available in packages for manipulating merkle search trees, but I want to suggest that this package include such functionality.