chore: refactor move DspRequestHandlerImpl

[wolf4ood]

What this PR changes/adds

• move DspRequestHandlerImpl into dsp-http-core module.
• move DspErrorResponse into dsp-http-spi module.

Why it does that

preparatory refactor for #3662

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cd0f8eb) 71.82% compared to head (10c0f60) 71.67%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3663      +/-   ##
==========================================
- Coverage   71.82%   71.67%   -0.16%     
==========================================
  Files         914      914              
  Lines       18375    18375              
  Branches     1042     1042              
==========================================
- Hits        13198    13170      -28     
- Misses       4717     4746      +29     
+ Partials      460      459       -1     

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