upgrade loses extra data in V2 -> V3 upgrade

[olsen232]

kart upgrade works by treating every dataset at every commit in the V2 repo as a datasource to be imported, presenting the same interface as it would if you imported from a GPKG or a shape file.

This is good from a reuse of code perspective and is likely to work fine for our users data. However, our own repos have extra data that is not generally importable from GPKG or similar, but which we need to preserve and is not currently being preserved. Specifically -

• attachments (eg license.txt, note this is not yet fully supported by kart)
• extra kart-specific metadata (generated-pks.json, which contains information to help us recognise new versions of existing features from pk-less datasources where we can't use the PK to do so).

Within the kart dataset tree, these blobs are found at the following places:

<dataset-path>/
    LICENSE.txt <-- attachment not be preserved by kart upgrade
    .sno-table/
        meta/
            title
            description
            schema.json
            generated-pks.json <-- extra metadata not preserved by kart upgrade