refactor(config): use dedicated files for "config" and "manager" code, and rename method receiver

[G-Rath]

While doing some deduplication for #2534, I found having these in the same file when they have the same receiver name to be confusing so I've moved them to different files and renamed the receiver for Manager to be m

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.92308% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.43%. Comparing base (4dfa15e) to head (6484d19).

Files with missing lines	Patch %	Lines
internal/config/manager.go	96.61%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2551      +/-   ##
==========================================
- Coverage   67.92%   67.43%   -0.49%     
==========================================
  Files         172      173       +1     
  Lines       13395    13492      +97     
==========================================
  Hits         9098     9098              
- Misses       3581     3678      +97     
  Partials      716      716              

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