[2/3]: Support Forwarding of Blinded Payments

[carlaKC]

This PR is the second in a series to add the ability for LND to forward payments to blinded routes.

⚠️ Lots of PR dependencies (do not fear the diff size!!) ⚠️

• Depends on: #8159
• Depends on: #8142

👀 First commit in this PR: hop: add function for calculating forwarding amount

Reviewer Notes:

• This PR does not signal the route blinding feature yet because it does not include error handling for blinded forwards, which is critical to receiver privacy so LND should not advertise the feature until it fully support it.
• Itests can be squashed, they're broken into multiple commits to aid readability.

Replaces #7376 Fixes #7298

[Roasbeef]

Will this be the last in the series to support forwarding?

[carlaKC]

Will this be the last in the series to support forwarding?

One more (WIP) to add and test error handling - it's quite delicate so I want to separate it out.

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[carlaKC]

Depending on the answer to my question in the first PR, this one might need to be updated to account for next_blinding_override.

Done!

[carlaKC]

Cross compilation error showing up on other PRs/ I can't reproduce locally so going to assume it's unrelated.

[guggero]

Cross compilation fails with: no space left on device... Oops, probably need to clear out some of the build caches.

EDIT: Cleared out build caches.

[carlaKC]

and then it doesnt know how to handle that due to 2 different htlc's coming in with the same hash

Not sure I'm following this. I can see that we're more at risk of malicious wormholing, but shouldn't we be able to handle that out of the box because we allow MPP payments with the same hash?

[carlaKC]

Last push is just a rebase to clear conflicts!

[carlaKC]

Added the ability to disable route blinding - works nicely in validation like @ellemouton suggested 🚀

[carlaKC]

We'd probably want to merge this only after https://github.com/lightningnetwork/lnd/pull/8485 is final?

I think that we could go ahead and merge this before https://github.com/lightningnetwork/lnd/pull/8485 is finalized because the content of forwarding vs error handling is pretty distinct in the codebase (ie, there are minimal interactions). Could also turn route blinding off by default and merge, then flip it back on in the next PR if we wanted to be extra cautious.

[carlaKC]

Added a469087932af89e7e38202ca3837789605eb4465 disable route blinding so that this can merge independently of #8485 (which will re-enable it + advertise feature).

[carlaKC]

Rebase! Also re-confirmed interop testing here.

[Roasbeef]

Needs a rebase!

[carlaKC]

Thanks for the review @Roasbeef, the suggested interface makes things much cleaner 🧹

Addressed some of your post-merge review of #8159 here (where relevant to the existing changes) and will follow with the rest either in #8485 or an independent followup.