cubicweb-wsme #4846397 Allow to fetch only some attributes [in-progress]

Currently we can only get all the attributes, or none (with the 'keyonly' flag).

The most compatible solution to this problem is to allow the use of 'fetch' together with 'keyonly':

GET /CWUser?keyonly=1&fetch=login

would return the user login in addition to its key and modification_date.

prioritynormal
typebug
done in<not specified>
closed by<not specified>
patchRemove the 'keyonly' feature. Rely only on 'fetch'. [in-progress]