Add Cosigner capabilities to the multisig

[tiero]

This change will need to modify the creation of multisig. At this stage, we need to provision spending limits and 2FA. It can be blocking.

This should refactor also the signing implementation to better separate domains.

When signing we need to decode the raw transaction and detect if m signatures provided (eg. 3 if it's a 3-of-5). If m-1 sigs provided we should check if also matching 2FA provided and satoshis to spent are less than the spending limit.

[lorenzodisidoro]

Supposition
I should save for each wallets to identify them and save more information (eg spending limits). So I can authenticate/authorize wallet and generate an otp for 2fa when it to do operations

Solution could be
Save wallet in a boltDB's bucket, identified with the wallet name used into Vault.
Save spending limit and other info into wallet bucket

[tiero]

The spending limits and other infos are per-multisig. What you point is the creation of the wallet of the Vault instance used to get a key pair to be used in a multisig, so not correct to save that.

If we call each multisig p2sh address the wallet it's more useful to save the scriptSigHash or the set of public keys provided in order to accept and authorize a future request