Add ability to send even numbered messages in sendcustommsg

[benthecarman]

It would be nice if you could send even numbered messages in sendcustommsg. I understand why it is currently disallowed as it gives some safety, but it can limit applications. For example, DLCs use even numbered messages and thus c-lightning can't be used for messaging.

Maybe a potential solution is to add a flag to c-lightning that allows for sending even numbered messages