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/config/RegistryConfig.java

@@ -1047,6 +1047,12 @@ public final class RegistryConfig {
       return config.registryPolicy.whoisCommandFactoryClass;
     }
 
+    @Provides
+    @Config("allocationTokenCustomLogicClass")
+    public static String provideAllocationTokenCustomLogicClass(RegistryConfigSettings config) {
+      return config.registryPolicy.allocationTokenCustomLogicClass;
+    }
+
     /**
      * Returns the header text at the top of the reserved terms exported list.
      *

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;

java/google/registry/config/files/default-config.yaml

@@ -41,6 +41,10 @@ registryPolicy:
   # See whois/WhoisCommandFactory.java
   whoisCommandFactoryClass: google.registry.whois.WhoisCommandFactory
 
+  # Custom logic class for handling allocation tokens.
+  # See flows/domain/token/AllocationTokenCustomLogic.java
+  allocationTokenCustomLogicClass: google.registry.flows.domain.token.AllocationTokenCustomLogic
+
   # Length of time after which contact transfers automatically conclude.
   contactAutomaticTransferDays: 5