Convert AuthenticationGenerator to plain module functions Use rustfmt for code formatting

[Hequin0x]

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 73.07692% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.79%. Comparing base (a011127) to head (d307180).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/generators/authentication_generator.rs	80.00%	8 Missing ⚠️
src/api/livebox_client.rs	0.00%	2 Missing ⚠️
src/commands/generate.rs	0.00%	1 Missing ⚠️
src/formatters/output_formatter.rs	0.00%	1 Missing ⚠️
src/renderers/authentication.rs	0.00%	1 Missing ⚠️
src/renderers/dhcp.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #5      +/-   ##
==========================================
- Coverage   45.96%   45.79%   -0.17%     
==========================================
  Files          11       11              
  Lines         322      321       -1     
==========================================
- Hits          148      147       -1     
  Misses        174      174              

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