Disposable key pair for fee payment

[grarco]

The client should support an optional flag to allow for the on-the-fly creation of a disposable keypair to sign a wrapper transaction and, therefore, pay fees. This would work in combination with a mandatory unshielding operation (required cli argument) releasing the entire (and exact) amount of tokens to cover for the fees. This way we can provide more privacy for the fee payer as the unshielding transfer is not directly linked to their address.