feat(chain): add spent_txouts and created_txouts methods to SPK and keychain indexes

feat(chain): add spent_txouts and created_txouts methods to SPK and keychain indexes

Implement sent_txouts and created_txouts methods on SpkTxOutIndex and
KeychainTxOutIndex. These methods return new SpentTxOut and CreatedTxOut
structs allowing callers to access complete transaction information
including script pubkeys, values, and spk index.