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)