Add support for showing multisig receiving address on Ledger

[reactive-relations]

Although Ledger wallet supports showing the address when receiving Bitcoin, and supports signing messages for sending Bitcoin from a multisig address, it doesn't support showing the address when receiving Bitcoin.

To implement this feature, all public keys must be sent in a command. One option for implementing this feature would be to implement the Get Public Key command (though it wouldn't be able to return a public key, just the constructed address).