Mappings for Volume annotation zarr streaming

[leowe]

Detailed Description

Follow-up to #6203 .

In #6195 we introduce a new field in the VolumeTracing proto object allowing to pin mappings. That is still a draft/prototype, but I assume we will merge something like that eventually, allowing to pick that mapping and request it for the fallback layer buckets. Of course, the datastore also needs to offer that parameter in some way, as tracked in #6193 .

Since #6203 introduces the possibility to stream volume annotations and their fallback layers, the pinned mappings need to respected there as well.