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.
      *