This class aggregates data required to spend transaction output.
For pay-to-address and pay-to-pubkey transactions it will have only a single key and CHECKSIG program as redeemScript.
For multisignature transactions there will be multiple keys one of which will be a full key and the rest are watch only,
redeem script will be a CHECKMULTISIG program. Keys will be sorted in the same order they appear in
a program (lexicographical order).