Spent txhash

[szist]

Currently the txo only includes a spent_at to verify if the utxo was spent or not.
I think it would be beneficial to also store the spent_tx BLOB to help identify the tx where the utxo was spent.

A use-case:
imagine a marketplace dapp. The dapp on the backend would have a matcher on the script address. Kupo would have all sell orders and their history and able to track the "active" sell orders. On the UI you don't really care about the buy transaction, only that the item was sold, but you would still want to provide a link to an explorer, where the transaction happened.