Calculate Trampoline onion packet sizes dynamically.

[arik-so]

Given that Trampoline onion sizes will be variable, this obviates the need to pass the onion size a priori.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 20 lines in your changes missing coverage. Please review.

Project coverage is 89.60%. Comparing base (66fb520) to head (b4da6fb). Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/onion_utils.rs	0.00%	20 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3333      +/-   ##
==========================================
- Coverage   89.66%   89.60%   -0.06%     
==========================================
  Files         126      126              
  Lines      102676   102766      +90     
  Branches   102676   102766      +90     
==========================================
+ Hits        92062    92088      +26     
- Misses       7894     7943      +49     
- Partials     2720     2735      +15     

Flag	Coverage Δ
	89.60% <0.00%> (-0.03%)	:arrow_down:

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[valentinewallace]

LGTM after squash!