Resource search is not correctly configured

[vsamtuc]

Resource search on extra fields is not correctly implemented.
The problem lies in the CKAN configuration.

From the docs:
If {field} is an extra field, then an attempt is made to match against
the extra fields stored against the Resource.

Note: The search is limited to search against extra fields declared in
the config setting ckan.extra_resource_fields.

We need to change the config of CKAN in order to support at least our basic
fields:
relation
package_type