Keysend feature bit announcement

[Ale1ster]

Background

c-lightning and eclair have implemented keysend, assigning the feature bit 54/55 for it.

Since it is finally adopted and implemented by the other daemons under the above feature bit pair, I think it would be better, for compatibility reasons at least, to announce it as a node feature as well.

Currently, when sending keysend payments from c-lightning v0.9.3 to lnd v0.11.1-beta, it fails due to the missing feature bit.
That is due to a strict requirement for the feature bit from c-lightning, but since it became adopted, wouldn't it be better if the corresponding feature was announced by the lnd nodes for compatibility?

As far as I have seen, the keysend feature bit has not been implemented yet, at least up until lnd v0.12.0-beta.

Keysend feature assignment proposal
c-lightning keysend to lnd error

c-lightning keysend MR
eclair keysend MR

[Roasbeef]

Our main focus right now is deploying AMP, which is a better version of keysend as it supports payment splitting, recurring push payments, a static invoice, and more. We wouldn't be opposed to a PR that adds in their experimental feature bit (though it's odd it's a lower feature bit).

[cfromknecht]

@Ale1ster see my response on the main issue: ElementsProject/lightning#4299 (comment)

The feature bit has not been standardized yet, and as @Roasbeef mentions keysend will be made obsolete in the near term by work on AMP.

[tvolk131]

@ellemouton hey I should have posted something here, but I started on the implementation for this yesterday... If you're already working on it then I'll abandon what I've done, but otherwise I can take this instead. Let me know!

[ellemouton]

hey @tvolk131 ! i'm about to push up a pr :) review welcome!

[tvolk131]

Sounds good! I just looked it over. I'm a new contributor, and it was reassuring to see that your code was basically identical to mine.