Multisig transaction support

[cmihai]

Multisignature transactions (M of N) are a common enough usecase that adding support for them to Unit-e should be a priority. As such, we need to implement two things:

1. Generating multisig addresses by providing the necessary public keys

2. Co-signing a multisig transaction, using keys stored either in hardware or in the wallet, and exporting the partially signed transaction to be signed by other parties.

Multisig transactions are not supported in Particl Desktop or in Particl Qt/Bitcoin Qt, so the users have to resort to raw RPC calls. An example of a wallet which supports multisig transactions is Electrum; however, it forces users to decide whether the entire wallet will be multisig or not, rather than individual transactions.