[RPC] add new utxoupdatepsbt arguments to the CRPCCommand and CPRCConvertParam tables

The new `descriptors` argument needs to be added to the Command and
ConvertParams tables to by usable as a named argument and by
bitcoin-cli.

Also update the test to use named arguments to test this.

src/rpc/client.cpp

@@ -28,6 +28,7 @@ public:
 static const CRPCConvertParam vRPCConvertParams[] =
 {
     { "setmocktime", 0, "timestamp" },
+    { "utxoupdatepsbt", 1, "descriptors" },
     { "generatetoaddress", 0, "nblocks" },
     { "generatetoaddress", 2, "maxtries" },
     { "getnetworkhashps", 0, "nblocks" },