multi: update GenTaprootFundingScript to pass tapscript root

In most cases, we won't yet be passing a root. The option usage helps us keep the control flow mostly unchanged.

graph/builder.go

@@ -1146,6 +1146,8 @@ func makeFundingScript(bitcoinKey1, bitcoinKey2 []byte,
 			return nil, err
 		}
 
+		// TODO(roasbeef): add tapscript root to gossip v1.5
+
 		return fundingScript, nil
 	}