add custom metadata mirroring

Currently, only a base set of metadata is mirrored, but the custom extra_field metadata is not.

Since these fields are extremely flexible, researchers must ensure a base set is maintained that corresponds to the metadata profile in Coscine.

The mapping code will assume the field names are identical (lower/upper case will be ignored).

Once metadata profiles generated in AIMS are supported in eLabFTW, this mirroring will need an update and should be more streamlined.