Instead of fetching a SigningProvider from ScriptPubKeyMan in order to fill and sign the keys and scripts for a PSBT, just pass that PSBT to a new FillPSBT function that does all that for us.
Instead of fetching a SigningProvider from ScriptPubKeyMan in order to fill and sign the keys and scripts for a PSBT, just pass that PSBT to a new FillPSBT function that does all that for us.