currencies in client never drifts from the server from now on

common/proto/table_definition.proto

@@ -743,4 +743,8 @@ message ListColumnTypesResponse {
 
   // Every column type, declarable or not, ordered by name.
   repeated ColumnType column_types = 1;
+
+  // Every canonical ISO-4217 code accepted by currency-bearing columns and
+  // profile accounting settings, ordered alphabetically.
+  repeated string currency_codes = 2;
 }