Support partial derivations in psbt

[stepansnigirev]

In some cases software wallets don't add a full derivation path to the psbt, but only last non-hardened derivation after known xpub. For example if normal derivation looks like m/84h/0h/0h/0/3 software that is using only xpub may provide only m/0/3 part. And fingerprint can be either a fingerprint of the xpub (btcpay) or a fingerprint filled with zeroes (blue wallet).
Makes sense to check if existing wallets can derive correct keys even with a short derivation. I think we can limit support to 2-index derivations.
I thought we had this supported but it looks like it's broken now, so also makes sense to add tests for it.