GH-5234: fix limited config support of FedX source selection cache

Add an additional constructor accepting a supplier for an initialized
cache.

tools/federation/src/main/java/org/eclipse/rdf4j/federated/cache/SourceSelectionMemoryCache.java

@@ -12,6 +12,7 @@
 
 import java.util.Map;
 import java.util.concurrent.ExecutionException;
+import java.util.function.Supplier;
 
 import org.eclipse.rdf4j.federated.endpoint.Endpoint;
 import org.eclipse.rdf4j.federated.exception.FedXRuntimeException;
@@ -49,6 +50,14 @@ public SourceSelectionMemoryCache(String cacheSpec) {
 		this.cache = CacheBuilder.from(CacheBuilderSpec.parse(cacheSpec)).build();
 	}
 
+	/**
+	 *
+	 * @param cacheSupplier provider for an instantiated Guava cache
+	 */
+	public SourceSelectionMemoryCache(Supplier<Cache<SubQuery, Entry>> cacheSupplier) {
+		this.cache = cacheSupplier.get();
+	}
+
 	@Override
 	public StatementSourceAssurance getAssurance(SubQuery subQuery, Endpoint endpoint) {