Sort LSPS2 opening fee params menu according to spec requirements

[martinsaposnic]

Closes #3462

The LSPS2 specification requires that the opening_fee_params_menu array be sorted in a specific order to simplify fee display for clients, but the current implementation returns parameters in an arbitrary order.

The LSPS2 spec:

The LSP, when ordering the opening_fee_params_menu array, MUST order by the following rules:

• The 0th item MAY have any parameters.
• Each succeeding item MUST, compared to the previous item, obey any one of the following:
  • Have a larger min_fee_msat, and equal proportional.
  • Have a larger proportional, and equal min_fee_msat.
  • Have a larger min_fee_msat, AND larger proportional.

This PR implements the sorting as described in the spec by sorting first by min_fee_msat, then by proportional for equal values.

The test creates an unsorted array of fee parameters and verifies that the returned menu follows all three ordering rules specified in the LSPS2 specification.

[ldk-reviews-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.83%. Comparing base (b8673f3) to head (16390e6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3840      +/-   ##
==========================================
- Coverage   89.88%   89.83%   -0.05%     
==========================================
  Files         162      162              
  Lines      130415   130422       +7     
  Branches   130415   130422       +7     
==========================================
- Hits       117217   117165      -52     
- Misses      10509    10552      +43     
- Partials     2689     2705      +16     

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[tnull]

LGTM

[tnull]

LGTM

Minor nit: please avoid rebasing on main if there are no conflicts, as it makes comparing the changes easier, especially if you force-push.

[ldk-reviews-bot]

🔔 1st Reminder

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