cubicweb #2410509 allow customization of RQL queries for FTI indexing [validation pending]

cubicweb.server.checkintegrity.reindex_entities fetches all entities that support FTI using very naive requests:

Any X WHERE X is TheEntityType

It would be better to at least prefetch indexable attributes since in most cases, the FTI process will need to access those attributes later. On big tables where several attributes are FT-indexed, this would significantly reduce the number of requests and improve performance. Furthermore, it would be nice for an entity class to be able to define a custom RQL query if for some reason, there's something more clever to do.

done in3.15.2
load left0.000
closed by#11063635c4e4 [fti] allow usage of custom RQL to fetch entities to index (closes #2410509)