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.

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

java/google/registry/config/RegistryConfigSettings.java

@@ -70,6 +70,7 @@ public class RegistryConfigSettings {
     public String productName;
     public String customLogicFactoryClass;
     public String whoisCommandFactoryClass;
+    public String allocationTokenCustomLogicClass;
     public int contactAutomaticTransferDays;
     public String greetingServerId;
     public List<String> registrarChangesNotificationEmailAddresses;