Support signing of 'change' UTXOs #2
Description
The API has been enhanced to support "change UTXOs", that is UTXOs with path like "1/x". This is needed to support imported wallets that have change addresses.
However, the signer does not support change UTXOs, it is not supplied with required information.
The API sig-reqs-by-dlc-ids contains UTXO signature requests, with one signer_index field. It has to be extended with signer_index4.
The cryptlib also has to be enhanced -- see CadenaWizard/cryptlib#17
The app should take the new signer_index4 parameter from the API and pass it to the cryptlib.