Change Stripe tax transaction exchange rate to BIGNUMERIC type. (#6428)

Some historical Stripe tax transactions contain exchange rates with 16 digits after the decimal, which exceeds the normal NUMERIC type's maximum scale of 9 digits after the decimal.

bigquery_etl/stripe/tax.transactions.itemized.1.schema.json

@@ -133,7 +133,7 @@
   },
   {
     "name": "filing_exchange_rate",
-    "type": "NUMERIC"
+    "type": "BIGNUMERIC"
   },
   {
     "name": "filing_total",

sql/moz-fx-data-shared-prod/stripe_external/itemized_tax_transactions_v1/schema.yaml

@@ -100,7 +100,7 @@ fields:
   type: STRING
   mode: NULLABLE
 - name: filing_exchange_rate
-  type: NUMERIC
+  type: BIGNUMERIC
   mode: NULLABLE
 - name: filing_total
   type: NUMERIC