Provide support for sidecar metadata file

ref @satra: would like to have a .tsv with spreadsheet with each row providing extra (or adjusted/fixed) metadata for a file within dandiset

- [ ] decide on placement, e.g. `/extra_metadata.tsv`
- [ ] adjust metadata loading to load from that file while possibly overloading metadata obtained from .nwb (should that be "default" or an option?)
- [ ] validator: we would need to annotate the "source of metadata": either it came from nwb, or from the sidecar to better alert user in case of invalid metadata

later
- establish mapping of (some) metadata fields back into nwb and provide a command like "adjust-nwb-metadata" which would take metadata from sidecar and place it within corresponding .nwb