Handle Purchase Order numbers explicitly in billing pipeline

Note that these aren't used for most registrars.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=213631863

java/google/registry/model/registrar/Registrar.java

@@ -310,6 +310,10 @@ public class Registrar extends ImmutableObject implements Buildable, Jsonifiable
   @Nullable
   Long billingIdentifier;
 
+  /** Purchase Order number used for invoices in external billing system, if applicable. */
+  @Nullable
+  String poNumber;
+
   /**
    * Map of currency-to-billing account for the registrar.
    *
@@ -422,6 +426,10 @@ public class Registrar extends ImmutableObject implements Buildable, Jsonifiable
     return billingIdentifier;
   }
 
+  public Optional<String> getPoNumber() {
+    return Optional.ofNullable(poNumber);
+  }
+
   public ImmutableMap<CurrencyUnit, String> getBillingAccountMap() {
     if (billingAccountMap == null) {
       return ImmutableMap.of();
@@ -644,20 +652,25 @@ public class Registrar extends ImmutableObject implements Buildable, Jsonifiable
       return this;
     }
 
-    public Builder setIanaIdentifier(Long ianaIdentifier) {
+    public Builder setIanaIdentifier(@Nullable Long ianaIdentifier) {
       checkArgument(ianaIdentifier == null || ianaIdentifier > 0,
           "IANA ID must be a positive number");
       getInstance().ianaIdentifier = ianaIdentifier;
       return this;
     }
 
-    public Builder setBillingIdentifier(Long billingIdentifier) {
+    public Builder setBillingIdentifier(@Nullable Long billingIdentifier) {
       checkArgument(billingIdentifier == null || billingIdentifier > 0,
           "Billing ID must be a positive number");
       getInstance().billingIdentifier = billingIdentifier;
       return this;
     }
 
+    public Builder setPoNumber(Optional<String> poNumber) {
+      getInstance().poNumber = poNumber.orElse(null);
+      return this;
+    }
+
     public Builder setBillingAccountMap(@Nullable Map<CurrencyUnit, String> billingAccountMap) {
       if (billingAccountMap == null) {
         getInstance().billingAccountMap = null;