Handle Purchase Order numbers explicitly in billing pipeline

Note that these aren't used for most registrars.

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

java/google/registry/beam/invoicing/sql/billing_events.sql

@@ -22,6 +22,7 @@ SELECT
   eventTime,
   BillingEvent.clientId AS registrarId,
   RegistrarData.accountId AS billingId,
+  RegistrarData.poNumber AS poNumber,
   tld,
   reason as action,
   targetId as domain,
@@ -63,6 +64,7 @@ JOIN (
   SELECT
     __key__.name AS clientId,
     billingIdentifier,
+    IFNULL(poNumber, '') AS poNumber,
     r.billingAccountMap.currency[SAFE_OFFSET(index)] AS currency,
     r.billingAccountMap.accountId[SAFE_OFFSET(index)] AS accountId
   FROM