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.

Backport of Core PR18115 part [6/9] : https://github.com/bitcoin/bitcoin/pull/18115/commits/82a30fade70a2a95c2bbeac4aa06dafda600479d

Depends on D8097