Add script_pubkey field to TxInput message

[andrewkozlik]

Signing Taproot UTXOs involves computing the new BIP341 hash, which hashes all previous scriptPubKeys, not just the one we are signing. If we own the input, then we can (and should) derive its scriptPubKey inside Trezor. If it's an external input, then we need the scriptPubKey to be supplied. We can already do that with the present API by requesting the corresponding PrevOutput, i.e. just the one previous output, not the entire previous transaction. An alternative is to change the API by adding a script_pubkey field to the TxInput message, which will have to be filled in for external inputs only. We have decided to go with the latter option. For the record, the reasons considered are summarized below.

Reasons to keep the API as is:

• The extra PrevOutput message will be needed only for external inputs (once for signature hashing and once for verification of externality). Apparently the only disadvantage is that in CoinJoin it will mean 2*(N-1) extra messages during signing, where N is the number of participants, which seems acceptable.
• The factor of size. By adding script_pubkey to TxInput we increase the maximum size of the message by another 520 bytes. TxInput is already the biggest message type we have with a theoretical maximum size of 7767 bytes and a practical maximum of ~4000 bytes.
• The PrevOutput solution seems nicer from a defensive coding point of view. we wouldn't need to check yet another optional field, we're guaranteed that an attacker isn't slipping in a value that gets accidentally used when it shouldn't, etc.

Reasons to add the script_pubkey field to the TxInput message:

• Fewer messages exchanged between host PC and Trezor.
• In the future this will make the API simpler for apps that use Trezor, because they won't have to implement PrevOutput message handling if they only sign Taproot inputs.
• In TxInput we already have fields like witness and ownership_proof which have very similar rules, e.g. ownership_proof may be present only for external inputs. The main difference between the addition of these fields and the question of adding script_pubkey is that there exists an alternative way to get script_pubkey via PrevOutput.
• TxInput already has the field amount, which also wouldn't have to be there and could be obtained via PrevOutput. The main difference between amount and script_pubkey is that amount is mandatory for every input whereas script_pubkey only for external inputs.
• Slightly simpler implementation of Taproot signing in legacy, where requesting the PrevInput means adding a new state to the state machine.