FedX - Allow for FedXConfig options to be overriden in repo config templates

[vtermanis]

Problem description

We (at Iotics) run RDF4J as a container and configure FedX at deploy time using a config template. It would be useful to be able to override defaults of FedXConfig options such as withEnforcemaxQueryTime and withSourceSelectionCacheSpec in the template.

Preferred solution

Be able to specify FedXConfig options in the template so it's possible e.g. change the maximum query time and source selection cache configuration, e.g.:

@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>.
@prefix rep: <http://www.openrdf.org/config/repository#>.
@prefix fedx: <http://rdf4j.org/config/federation#>.

[] a rep:Repository ;
   rep:repositoryImpl [
      rep:repositoryType "fedx:FedXRepository" ;
      fedx:config [
         fedx:enforceMaxQueryTime 42 ;
         fedx:enableMonitoring true ;
      ];
   ];
   rep:repositoryID "my-federation" ;
   rdfs:label "FedX Federation" .

Are you interested in contributing a solution yourself?

Yes

Alternatives you've considered

Because FedXRepositoryConfig already handles FedX repo config templates, we have not considered any alternatives.

Anything else?

We don't know whether it makes sense to expose all or just a subset of the available options. (See #4953)