Introduce MultisigKeyBag to expose P2SH redeem data

For married wallets KeyChainGroup now keeps redeem scripts together with keys and exposes this data via MultisigKeyBag interface. In future this interface may be used by signers to get data required for signing P2SH transactions.

The "right" way to do married wallets is imho 2-of-3 scripts in which the user has two keys (one online and one offline) and the risk analysis service has one. So this would be normal, I think, not unexpected.