Generalise spatialdata_software_version into a provenance field #12

This issue was AI-generated—manually reviewed.

The root attribute spatialdata_attrs.spatialdata_software_version assumes that every store is written by the Python spatialdata package. Other implementations, possibly in other languages, also write SpatialData stores.

For completeness, the store could record the package that wrote it and its version, for example in a provenance field. The specification text stays unchanged for now (spec/on-disk/container.md, "Root group").