Convert Boost JSON string to std::string_view

[knoepfel]

The fmt::format(...) function can represent a Boost JSON string as an array of chars instead of character string (e.g.):"

Both 'cpp' and 'py' parameters specified for malformed3
    - cpp: ['m', 'y', '_', 'o', 't', 'h', 'e', 'r', '_', 'p', 'y', 't', 'h', 'o', 'n', '_', 'p', 'h', 'l', 'e', 'x', '_', 'm', 'o', 'd', 'u', 'l', 'e']

This PR makes use of Boost JSON string's implicit conversion operator to std::string_view so that fmt::format(...) will form the output correctly (e.g.):

Both 'cpp' and 'py' parameters specified for malformed3
    - cpp: my_other_python_phlex_module

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

❌ Your project status has failed because the head coverage (76.50%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #225      +/-   ##
==========================================
- Coverage   76.61%   76.50%   -0.11%     
==========================================
  Files         124      124              
  Lines        2733     2733              
  Branches      475      475              
==========================================
- Hits         2094     2091       -3     
- Misses        446      447       +1     
- Partials      193      195       +2     

Flag	Coverage Δ
unittests	76.50% <100.00%> (-0.11%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
phlex/app/load_module.cpp	85.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

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[pcanal]

LGTM (also verified it solves the issue in my configuration).