Add a way to register an interposed synchronization without associating the txn

It would be very useful to the application server if there were an API method to register an interposed synchronization on a transaction that was not currently associated with the calling thread, the same way that non-interposed synchronizations can be. Right now the only two options are to use TransactionSynchronizationRegistry, which requires that the existing transaction be suspended and a new one associated, or to use reflection to directly call the registerSynchronizationImple method of the transaction. Neither one is optimal.