addinvoice: provide hop hints for no-amount invoice

[bodhike]

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Change Description

This change allows creating no-amount private invoices that do provide hop hints, up to maxHopHints.

Fixes #6738

Steps to Test

The testInvoiceRoutingHints itest was expanded to account for this use case. If someone can provide some pointers on how to easily decode the payment request response from lncli addinvoice, I'd be glad to provide an example from the end-user's perspective.

This itest and the added unit test case fail before the change to addinvoice.go (given that change was so minimal, I clubbed it together in the same commit with the unit test change, but I can separate them if deemed necessary :slightly_smiling_face: ).

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Testing

• [ ] Your PR passes all CI checks.
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[bodhike]

Thanks for the prompt review @positiveblue :pray: I've pushed a fixup for merging the ifs and a separate commit for the use require refactoring :boom:

[bodhike]

Thanks @joostjager, I've rebased and rearranged commits, putting first the refactorings and typo fixes. The last three commits contain the functional change, new unit test case and new itest case, respectively.

[joostjager]

Kicked off CI - perhaps there are linter issues

[bodhike]

@joostjager It appears the linter job succeeded. OTOH most of the (old?) itests are also failing in master. Is there a particular issue I should pay attention to?

[joostjager]

I am not sure what the status of the itests is. I see them failing a lot generally.

[bodhike]

@yyforyongyu I've pushed again addressing your feedback. LMKWYT.

[bodhike]

Thanks @yyforyongyu! I've updated the release notes in the latest commit, and I'll keep an eye open on #7126 :+1:

[guggero]

Needs a rebase because of a conflict in the release notes.

[bodhike]

I've rebased and fixed the conflicts again, and the itests are a little better now :+1: :pray: