dumpers: implement RelationDumperExt.load(...)

[slint]

When loading a record with relations from an ES response, the (partially) dumped relations, should be loaded into their relation record class and be cached (e.g. to avoid accidental DB queries when rendering search results).

This has to be done in the RelationsField (e.g. in RelationResult.__call__), and depending on if the attrs of the field are present in the record data, one can assume that there is no need to resolve the relation ID (i.e. fetch from the DB).