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.

It worth having it separate only if we need to keep existing "KeyChain is a KeyBag" relationship. Otherwise, they better be merged.