Metadata upload does not set custom fields

[etj]

The parser/storer design allows for metadata schema extendibility, so that plugins may be able to extract custom fields from the uploaded metadata and store them in a customized way.

The metadata upload API endpoint is not using the custom parsed fields:

• https://github.com/GeoNode/geonode/blob/4.4.3/geonode/layers/api/views.py#L148

and it's not passing them to the update_resource function`

• https://github.com/GeoNode/geonode/blob/4.4.3/geonode/layers/api/views.py#L156