Scaffold message with empty placeholder

[lumtis]

Command to scaffold a message with arguments but no logic inside. Leave an empty placeholder, the same as for packet you have everything scaffolded, you just need to add the logic.

starport message foo field1 field2... --module [...]

appd tx app foo field1 ....

Would be necessary for the exchange tutorial because removing orders involves having a new message without new types or packets and there is pretty much a lot of generic code to add manually