segment-to-bucket index for volume annotations *with fallback segmentation*

[fm3]

Follow-up for #7063 with fallback segmentation layer. This requires segment index metadata for the fallback layer to be available. Format to be determined.

First step would be to read the segment index file in the datastore (similar to the meshfile).
Second step is to then enable the segment index feature also for volume annotations with fallback layer, mixing the info from datastore and tracingstore.

We need to make sure the delete segment action does not explode by tackling hundreds of thousands of buckets

[fm3]

First version of the segment index file now available, compare https://scm.slack.com/archives/C03B6160ATY/p1697730414324389 and https://github.com/scalableminds/voxelytics/pull/3347