Require metadata fields by files or datasets

[lmarini]

We currently let users specify whether a metadata field is required or not but we don't let them specify whether to require those on files or datasets. They are either required on both or none. We should let the user pick which type they are required for.

In the metadata definition we set "required" : true. Maybe it can be an array of resource types this is required for. If empty, then it's not required. For example "required:["file"] or "required:[] or "required:["file","dataset"].

In the future we will do this by space/collection/dataset.