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.

Is it worth having this be a separate interface? I noticed quite a few methods now require a MultisigKeyBag instead of just a KeyBag. Perhaps they should be merged? What do you think?