Avoid wrapping genesis txs

[grarco]

Right now in the tx_to_sign method of the TxToSign trait we call get_tx_to_sign which produces a wrapper transaction. We don't need a fee payer for a genesis transaction, and indeed the signature on it is produced and checked only against the raw_headher_hash: we should avoid wrapping the raw transaction in the first place.

Doing so would also remove the need for the genesis_fee_payer_pk and genesis_fee_token_address functions. Because the fee payer is required in the SigningTxData we need for signing, this seems to depend on #3901.

We could also think about adding an expiration to the genesis tx back in the past (we could use the same value we are using for the timestamp right now), just to make it even more strong against possible replays after genesis: this last change though would brake any genesis process currently underway.

[grarco]

Right now it is not possible to apply the changes suggested here because the hardware wallet can only handle wrapped transactions. We could come back at this if we ever changed that logic, for now I'm setting the priority of this issue to low