Convert Authorize.Net transaction id data type to String

This prepares the API for consistent transaction data types between Authorize.Net with forthcoming Trust Commerce integration (whose transaction id's must need be strings).

This PR could've come as part of the future Trust Commerce PR, but considering that integration may not be available until after EventHub has gone live, although it still may have been technically feasible to alter the database on a live application, it seemed safer to do so in advance.