Make BillingModule.OVERALL_INVOICE_PREFIX configurable

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

java/google/registry/beam/invoicing/InvoicingPipeline.java

@@ -75,6 +75,10 @@ public class InvoicingPipeline implements Serializable {
   @Config("billingBucketUrl")
   String billingBucketUrl;
 
+  @Inject
+  @Config("invoiceFilePrefix")
+  String invoiceFilePrefix;
+
   @Inject
   InvoicingPipeline() {}
 
@@ -164,7 +168,7 @@ public class InvoicingPipeline implements Serializable {
                         billingBucketUrl,
                         BillingModule.INVOICES_DIRECTORY,
                         yearMonth,
-                        BillingModule.OVERALL_INVOICE_PREFIX,
+                        invoiceFilePrefix,
                         yearMonth)))
         .withHeader(InvoiceGroupingKey.invoiceHeader())
         .withoutSharding()