Support partial restore of only certain objects without reading the entire stream

[anelson]

When restoring a tar archive from stdin, it's not possible to seek, and therefore the entire archive must be read even if the user wants to restore only a small subset of objects. However, for file and S3-based archives this isn't the case, and a seekable interface could be presented, so that partial restores can be more efficient.

The challenge to doing this for archives in object storage is that some intelligent buffering is still needed to avoid an excessive amount of GET operations being performed as the tar crate reads bits of metadata here and there.