VRT: allow pinning sources to a specific GCS object generation

[alexberndt]

Feature description

Summary

A VRT references each source by path only. There is no way to record a specific GCS object generation (version) for a source, so opening a VRT always reads the live (current) version of each /vsigs/ source. If a source object is later overwritten (with GCS object versioning enabled), the same VRT silently starts reading different data.

Why this matters

For reproducibility and provenance, a VRT built at a given time should be able to keep referring to the exact object versions it was composed with, independent of later writes to those paths. Today that is impossible: the version information has nowhere to live in the VRT.

Current behaviour

• GDAL's /vsigs/ handler reads the live object. Once /vsigs/: Handle Query-Based Generation Params to Support GCS Object Versioning #14947 is merged and released, a specific generation will be able to be requested via the GS_GENERATION path-specific option (see also /vsigs/: support reading a specific object generation (GCS object versioning) #14946). However, the caller has to set that option in code before opening the source; today it cannot be stored in the VRT.
• The VRT format has no per-source field to carry a generation: VRTSimpleSource::XMLInit / SerializeToXML in frmts/vrt/vrtsources.cpp parse and write relativeToVRT and shared on <SourceFilename>, but nothing for an object version, so a generation cannot be persisted or applied when the source opens.

[rouault]

An optional per-source generation field on <SourceFilename>, e.g.:

I'm -1 on adding VSI specific options to VRT. If the generation number needs to be captured, it should be captured in the VSI path itself. Like "/vsigs?generation=xxxxxxxx&path=bucket/img.tif"

[alexberndt]

Sure, that might actually simplify things! Let me implement this in #14947 before we merge that then.