psbt: have wally_tx serialization methods be legible for gen'd code

our code generators expect the serialization name to match the struct
type

tools/generate-wire.py

@@ -235,6 +235,7 @@ class Type(FieldSet):
         'onionmsg_path',
         'route_hop',
         'tx_parts',
+        'wally_psbt',
     ]
 
     # Some BOLT types are re-typed based on their field name