Bound Persister's WriteableScore by Deref

[TheBlueMatt]

This makes the trait marginally more flexible, but more importantly matches our normal structure which makes the bindings generator a bit happier.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.08%. Comparing base (f89e963) to head (636125e).
Report is 9 commits behind head on main.

Files	Patch %	Lines
lightning-background-processor/src/lib.rs	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3260      +/-   ##
==========================================
+ Coverage   89.72%   90.08%   +0.36%     
==========================================
  Files         125      125              
  Lines      102801   104593    +1792     
  Branches   102801   104593    +1792     
==========================================
+ Hits        92234    94222    +1988     
+ Misses       7859     7686     -173     
+ Partials     2708     2685      -23     

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