Make BillingModule.OVERALL_INVOICE_PREFIX configurable

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

java/google/registry/config/RegistryConfig.java

@@ -692,6 +692,18 @@ public final class RegistryConfig {
       return ImmutableList.copyOf(config.billing.invoiceEmailRecipients);
     }
 
+    /**
+     * Returns the file prefix for the invoice CSV file.
+     *
+     * @see google.registry.beam.invoicing.InvoicingPipeline
+     * @see google.registry.reporting.billing.BillingEmailUtils
+     */
+    @Provides
+    @Config("invoiceFilePrefix")
+    public static String provideInvoiceFilePrefix(RegistryConfigSettings config) {
+      return config.billing.invoiceFilePrefix;
+    }
+
     /**
      * Returns the Google Cloud Storage bucket for staging escrow deposits pending upload.
      *

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). */

java/google/registry/config/files/default-config.yaml

@@ -293,6 +293,7 @@ icannReporting:
 
 billing:
   invoiceEmailRecipients: []
+  invoiceFilePrefix: REG-INV
 
 rde:
   # URL prefix of ICANN's server to upload RDE reports to. Nomulus adds /TLD/ID