Tracking bug for metadata-only-import from S3 or other hosted storage

[olsen232]

A yet-to-be-fully-designed flow that creates a tile-based Kart dataset that is "backed" in some sense by the original data on S3 (or similar), rather than being necessarily stored in the local Kart repo or available-on-demand from the remote Kart repo. The storage format will be almost identical to a regular tile-based Kart dataset, except that extra information will be stored about the original location where the tile was found, so it can be fetched from there if necessary.