NoSQL: expose connection-string

Author: snazy

components/persistence/db/mongodb/src/testFixtures/java/org/apache/polaris/persistence/mongodb/MongoDbBackendTestFactory.java

@@ -57,6 +57,10 @@ public Optional<String> databaseName() {
     return factory.buildBackend(config);
   }
 
+  public String connectionString() {
+    return connectionString;
+  }
+
   @Override
   public void start() {
     start(Optional.empty());