Improve the tx submission reponse

Currently in the `cardano-tx-submit-webapi` I use a pair ot `StrictTMVar`s to submit a transaction to the network and get the response. The type of the response is `Maybe ApplyMempoolPayloadErr` which is returns either an error in the `Just` or `Nothing` if the submission is successful.

For something like the `cardano-tx-submit-webapi` for a succesful submission, I would like return the transaction id to the caller. With the current types that is not possible.