Fixes#6396.
This commit fixes a panic that occurred when trying to sign for a
Taproot output without specifying the full UTXO information for each
input. Instead of panicking an error is now returned.
Because Taproot key spend only spends don't allow us to re-construct the
spent pkScript from the witness alone, we cannot support registering
spend notifications for v1 pkScripts only. We instead require the
outpoint to be specified. This commit makes it possible to only match by
outpoint and also adds an itest for it.
