Allow plugins to contribute Prolog predicates

A new extension point is defined that allows plugins to provide
additional packages that contain Prolog predicates. The predicates
can e.g. be used in the project submit rules.

Change-Id: I097a9671d6e740755252fb494e75e44dfd3c3d49
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-server/src/main/java/com/google/gerrit/rules/PrologModule.java

@@ -14,11 +14,13 @@
 
 package com.google.gerrit.rules;
 
+import com.google.gerrit.extensions.registration.DynamicSet;
 import com.google.gerrit.server.config.FactoryModule;
 
 public class PrologModule extends FactoryModule {
   @Override
   protected void configure() {
+    DynamicSet.setOf(binder(), PredicateProvider.class);
     factory(PrologEnvironment.Factory.class);
   }
 }