Add support for gasless transactions to Transact Step

[efstajas]

We are thinking about sponsoring some transactions on FVM entirely since gas cost is so low. When sponsoring a non-anonymous transaction, we still need to prompt the user to sign a message in their wallet, but not execute a transaction.

For this, we should add a new mode to the new Transact Step that shows transaction's gas as "Paid for by Drips", and prompts the user to "Sign in wallet" instead of sending a transaction.