Zarr backend followup: OME-NGFF, user-side improvements

[fm3]

Follow-up for reading zarr images in wk-backend backend (#6019)

• handle multi-layers/ngff
  • handle OME-NGFF metadata Import Remote Zarr Datasets #6335
  • handle non-standard axis order (as specified by ome metadata?) Import Remote Zarr Datasets #6335
• pass through channels all the way to the front-end
• expose as first-class feature to users
  • adapt frontend compatibility detection for upload
  • routes to add remote datasets Import Remote Zarr Datasets #6335
  • adapt documentation
  • Store Credentials on wk side in db rather than in datasource-properties.json
  • Do remote datasets need a datasource-properties.json at all?

[normanrz]

[ ] pass through channels all the way to the front-end

This could also be solved by adding a channelIndex field to the datasource metadata, which allows to cut out different channels of a dataset. Then, no frontend changes would be required.

[fm3]

The remaining todos are tracked in individual issues now: #6578 and #6525