Construct Trampoline onion from path.

[arik-so]

No description provided.

[TheBlueMatt]

Shouldn't this be a different variant? ISTM it would clean up the accessing code if we call this a TrampolineForward because its really a forward, not have forwarding logic in handling the Receive variant.

[arik-so]

I have moved it to a separate variant, which indeed makes perfect sense.

[TheBlueMatt]

Then it should be in a separate struct, not in the same enum, no?

[arik-so]

This is still an onion payload, just in this case for an inner onion. The last payload will still be the Receive variant. I'm not 100% sure it makes sense to create a whole separate struct for that.

[arik-so]

Thanks for the feedback, @TheBlueMatt! I'll update #2756 to reflect your recommendations.

[valentinewallace]

Could we rebase this on #2906? Hard to evaluate the new OutboundTrampolinePayload out of context

[arik-so]

Yeah, ish, but absent a Receive variant, it's gonna completely break, so I think once 2906 is in I'll need to rework this. Bastien has been very helpful with trampoline to blinded path serialization references, so I'll alter this PR to do that instead.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 30.41475% with 151 lines in your changes are missing coverage. Please review.

Project coverage is 90.07%. Comparing base (ac9a2c8) to head (e768f53).
Report is 39 commits behind head on main.

Files	Patch %	Lines
lightning/src/ln/onion_utils.rs	0.86%	114 Missing ⚠️
lightning/src/ln/msgs.rs	50.94%	22 Missing and 4 partials ⚠️
lightning/src/routing/router.rs	56.25%	7 Missing ⚠️
lightning/src/events/mod.rs	25.00%	3 Missing ⚠️
lightning-background-processor/src/lib.rs	0.00%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2927      +/-   ##
==========================================
+ Coverage   89.42%   90.07%   +0.64%     
==========================================
  Files         117      118       +1     
  Lines       96290   104861    +8571     
  Branches    96290   104861    +8571     
==========================================
+ Hits        86109    94454    +8345     
- Misses       7962     8246     +284     
+ Partials     2219     2161      -58     

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