chore: make Bolt11InvoiceDescriptionRef copy

[elsirion]

Just a minor API paper cut fix, will lead to less .clone()ing.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.65%. Comparing base (797993c) to head (64a3bd6).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3445      +/-   ##
==========================================
- Coverage   89.69%   89.65%   -0.05%     
==========================================
  Files         130      130              
  Lines      107335   107265      -70     
  Branches   107335   107265      -70     
==========================================
- Hits        96277    96166     -111     
- Misses       8658     8689      +31     
- Partials     2400     2410      +10     

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