WIP: Warn about unused settings in broker.conf

[adombeck]

WIP because I didn't test it yet

We might also want to use a map for defining the sections and keys so that we don't have to keep the list of variables in sync with the new map.

TODO

• add tests

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.10%. Comparing base (49ac33a) to head (cbbfdd2).

Files with missing lines	Patch %	Lines
internal/broker/config.go	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #706      +/-   ##
==========================================
- Coverage   82.14%   82.10%   -0.04%     
==========================================
  Files          21       21              
  Lines         997     1006       +9     
==========================================
+ Hits          819      826       +7     
- Misses        178      180       +2     

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