Enable use of external fmt

[mtao]

Current code assumes the use of spdlog's bundled fmt even thought dependencies might overwrite this behavior.

fmt/ostream.h / spdlog/fmt/ostr.h seems to break for me - so i removed their usage for now with some utility functions

[codecov]

Codecov Report

Attention: Patch coverage is 51.72414% with 14 lines in your changes missing coverage. Please review.

Project coverage is 81.59%. Comparing base (0cd21ba) to head (26702a7).

Files with missing lines	Patch %	Lines
src/polysolve/nonlinear/Criteria.cpp	33.33%	12 Missing ⚠️
src/polysolve/nonlinear/Solver.cpp	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   81.20%   81.59%   +0.38%     
==========================================
  Files          49       49              
  Lines        2001     1994       -7     
  Branches      267      267              
==========================================
+ Hits         1625     1627       +2     
+ Misses        376      367       -9     

Flag	Coverage Δ
polysolve	81.59% <51.72%> (+0.38%)	⬆️

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