This adds a descriptors argument to the utxoupdatepsbt RPC. This means:
- Input and output scripts and keys will be filled in when known
- P2SH-witness outputs will be filled in from the UTXO set when a descriptor is provided to show they're segwit outputs.
https://github.com/bitcoin/bitcoin/pull/15427/commits/26fe9b990995f9cb5eee21d40b4daaad19f7181f
Depends on D6624
Concludes backport of Core PR15427