Feature: Keysend custom tlv

[Evanfeenstra]

Adds send_with_custom_tlv to SpontaneousPayment, so you can send a keysend with custom data inside.

Please LMK if there is more bindings stuff or testing that should be added here.

Thanks!

[tnull]

Thank you for looking into this! Already looks pretty good to me, just a few comments.

I think it would make sense to also add custom TLV support of the receiver's end while we're at it, which would also allow adding end-to-end test coverage in unit tests.

[tnull]

Thanks for adding the receive part also. I don't think we need / should add the TlvEntries wrapper type, but apart from that it mostly looks good.

Btw, please try to clean up the the commit history so that it reflects a number of logical changes to the codebase. Fixups should be 'sorted in' beneath such feature commits so they can be squashed into the respective commits before merging.

[Evanfeenstra]

@tnull after those last updates, the changes are small enough that I just squashed to a single commit. This is much cleaner now :)

[tnull]

@tnull after those last updates, the changes are small enough that I just squashed to a single commit. This is much cleaner now :)

Thanks, but note that it's usually preferable to break up a PR into multiple commits and to squash fixups in rounds or just before merging as it allows reviewers to follow what comments/requested changes were already addressed.

[tnull]

LGTM, please squash the fixup commit.