create default search string filter for new entity list endpoint

[sennierer]

in relation to #850 and corresponding PR #851
Using the get_entity_classes utility function we could inspect the classes and - using sub-queries and annotations - concatenate all charfields in one field, which we could then use to create a basic label filter for the entities list endpoint. This would allow to without changing anything have a basic search entities endpoint.

[b1rger]

Something similar is being done in

apis-core-rdf/apis_core/generic/helpers.py

Line 11 in 8da1372

def generate_search_filter(model, query, fields_to_search=None):