Support multisig inputs on the Ledger Nano

[cmihai]

Is your feature request related to a problem? Please describe Right now, we disallow multisig transactions that mix hardware and wallet keys, because we'd have to switch between TransactionSignatureCreator and DeviceSignatureCreator based on the key needed to sign, and the ProduceSignature function does not have the logic to do this.

Describe the solution you'd like We can create a proxy class that will encapsulate both signature creators, and will dynamically choose one of them based on the metadata of the key used to sign.