Store a reference to an allocation token in the OneTime billing event

We will need to be able to find all redemptions associated with a particular token and this will allow us to do that.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=245292865

java/google/registry/flows/domain/DomainCreateFlow.java

@@ -295,6 +295,7 @@ public class DomainCreateFlow implements TransactionalFlow {
             years,
             feesAndCredits,
             historyEntry,
+            allocationToken,
             now);
     // Create a new autorenew billing event and poll message starting at the expiration time.
     BillingEvent.Recurring autorenewBillingEvent =
@@ -477,6 +478,7 @@ public class DomainCreateFlow implements TransactionalFlow {
       int years,
       FeesAndCredits feesAndCredits,
       HistoryEntry historyEntry,
+      Optional<AllocationToken> allocationToken,
       DateTime now) {
     ImmutableSet.Builder<Flag> flagsBuilder = new ImmutableSet.Builder<>();
     // Sunrise and anchor tenancy are orthogonal tags and thus both can be present together.
@@ -497,6 +499,7 @@ public class DomainCreateFlow implements TransactionalFlow {
         .setPeriodYears(years)
         .setCost(feesAndCredits.getCreateCost())
         .setEventTime(now)
+        .setAllocationToken(allocationToken.map(Key::create).orElse(null))
         .setBillingTime(
             now.plus(
                 isAnchorTenant

java/google/registry/model/billing/BillingEvent.java

@@ -40,6 +40,7 @@ import google.registry.model.annotations.ReportedOn;
 import google.registry.model.common.TimeOfYear;
 import google.registry.model.domain.GracePeriod;
 import google.registry.model.domain.rgp.GracePeriodStatus;
+import google.registry.model.domain.token.AllocationToken;
 import google.registry.model.reporting.HistoryEntry;
 import google.registry.model.transfer.TransferData.TransferServerApproveEntity;
 import java.util.Objects;
@@ -244,6 +245,11 @@ public abstract class BillingEvent extends ImmutableObject
      */
     Key<? extends BillingEvent> cancellationMatchingBillingEvent;
 
+    /**
+     * The {@link AllocationToken} used in the creation of this event, or null if one was not used.
+     */
+    @Index @Nullable Key<AllocationToken> allocationToken;
+
     public Money getCost() {
       return cost;
     }
@@ -264,6 +270,10 @@ public abstract class BillingEvent extends ImmutableObject
       return cancellationMatchingBillingEvent;
     }
 
+    public Optional<Key<AllocationToken>> getAllocationToken() {
+      return Optional.ofNullable(allocationToken);
+    }
+
     @Override
     public Builder asBuilder() {
       return new Builder(clone(this));
@@ -306,6 +316,11 @@ public abstract class BillingEvent extends ImmutableObject
         return this;
       }
 
+      public Builder setAllocationToken(@Nullable Key<AllocationToken> allocationToken) {
+        getInstance().allocationToken = allocationToken;
+        return this;
+      }
+
       @Override
       public OneTime build() {
         OneTime instance = getInstance();