It's undocumented and only used in one place, so we can change it (it
was new in 24.08).
We really want to be able to just handle a raw onionmessage: this allows
oblivious sending of messages, but also, in combination with the coming
onionmessage_forward_fail notification, allows us to connect then
reinject.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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