Rework wrapper signature workflow

[grarco]

We define in the SDK SigningTxData, which is a support type to sign transactions, as follows:

https://github.com/anoma/namada/blob/7ac415a68791245e15c22ba551b1fddb301dfbcb/crates/sdk/src/signing.rs#L65-L76

After #3883 and #3900 it seems like the fee_payer field is too limited. We don't always want to sign the wrapper transaction or sometimes we might have an offline signature that we'd want to use instead of producing a new one. This seems to lead to some workarounds in the codebase to allow these different workflows: we should think about reworking this piece of data to support these cases (no signature or load a signature).

Also, as a minor note, there are a couple of function (tx_signers and aux_signing_data) that specifically handle the MASP case: we should see if we can collapse the MASP case to some other case to avoid this specific handling (like not passing an owner when the source of a tx is the MASP)

[grarco]

In addition, we should also review the signatures and wrapper_signature fields of args::Tx. We should be using these only when loading a tx from storage (with the tx command), so maybe it would be better to take them out of the general Tx args and move them into the specific args for TxCustom (see #4620)