Improve on offline signing

[grarco]

There are a couple shortcomings in the current implementation of offline signing:

1. Producing a signature with the hardware wallet is not supported
2. Producing a MASP signature is not supported

To address the first point we should modify the SignOffline struct to accept a vector of PublicKeys instead of SecretKeys so that we could use those to call the with_hardware_wallet function.
For the second point we should rework the way we sign masp transactions since at the moment we sign them after having generated them and before calling sign_tx (meaning we sign them even if we dump the transaction).

While addressing the points here above we should also see if it is possible to share more code between the sign_tx function and the generate_tx_signatures one.