FillPSBT will add our own scripts to the PSBT if those inputs are ours.
If an input also lists pubkeys that we happen to know the private keys
for, we will sign those inputs too.
Backport of Core PR16528 [30/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/72a9540df96ffdb94f039b9c14eaacdc7d961196