Add extensibility framework for allocation tokens

This uses an extensibility mechanism similar to that of WhoisCommandFactory
and CustomLogicFactory, namely, that a fully qualified Java class is
specified in the YAML file for each environment with the allocation token
custom logic to be used.  By default, this points to a no-op base class
that does nothing.  Users that wish to add their own allocation token
custom logic can simply create a new class that extends
AllocationTokenCustomLogic and then configure it in their .yaml config
files.

This also renames the existing *FlowCustomLogic *Flow instance variables
from customLogic to flowCustomLogic, to avoid the potential confusion with
the new AllocationTokenCustomLogic class that also now exists.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=183003112

java/google/registry/flows/domain/token/AllocationTokenModule.java

@@ -0,0 +1,33 @@
+// Copyright 2017 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.flows.domain.token;
+
+import static google.registry.util.TypeUtils.getClassFromString;
+import static google.registry.util.TypeUtils.instantiate;
+
+import dagger.Module;
+import dagger.Provides;
+import google.registry.config.RegistryConfig.Config;
+
+/** Dagger module for allocation token classes. */
+@Module
+public class AllocationTokenModule {
+
+  @Provides
+  static AllocationTokenCustomLogic provideAllocationTokenCustomLogic(
+      @Config("allocationTokenCustomLogicClass") String customClass) {
+    return instantiate(getClassFromString(customClass, AllocationTokenCustomLogic.class));
+  }
+}