Make BillingModule.OVERALL_INVOICE_PREFIX configurable

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=221700223

java/google/registry/config/RegistryConfigSettings.java

@@ -131,6 +131,7 @@ public class RegistryConfigSettings {
   /** Configuration for monthly invoices. */
   public static class Billing {
     public List<String> invoiceEmailRecipients;
+    public String invoiceFilePrefix;
   }
 
   /** Configuration for Registry Data Escrow (RDE). */