feat: return APIError from ReloadDeclarativeRawConfig

[czeslavo]

Makes ReloadDeclarativeRawConfig return an APIError when the status code is <200 or >=400.
KIC needs to inspect the HTTP response status code when invoking ReloadDeclarativeRawConfig to decide whether it's a recoverable issue (4xx) or not (5xx).

Required for Kong/kubernetes-ingress-controller#6237.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.57%. Comparing base (992aa54) to head (99cb2b6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #452      +/-   ##
==========================================
+ Coverage   59.41%   59.57%   +0.15%     
==========================================
  Files          71       71              
  Lines        4408     4408              
==========================================
+ Hits         2619     2626       +7     
+ Misses       1174     1168       -6     
+ Partials      615      614       -1     

Flag	Coverage Δ
2.1	35.82% <0.00%> (ø)
2.2	48.34% <0.00%> (ø)
2.3	48.97% <0.00%> (ø)
2.4	49.02% <0.00%> (ø)
2.5	49.02% <0.00%> (ø)
2.6	49.02% <0.00%> (ø)
2.7	50.72% <0.00%> (ø)
2.8	50.72% <0.00%> (ø)
3.0	54.85% <100.00%> (+0.20%)	⬆️
3.1	56.48% <100.00%> (+0.20%)	⬆️
3.2	56.48% <100.00%> (+0.15%)	⬆️
3.3	56.48% <100.00%> (+0.15%)	⬆️
3.4	58.87% <100.00%> (+0.15%)	⬆️
3.5	56.66% <100.00%> (+0.15%)	⬆️
3.6	56.66% <100.00%> (+0.15%)	⬆️
community	43.62% <100.00%> (+0.15%)	⬆️
enterprise	58.03% <0.00%> (ø)
integration	59.57% <100.00%> (+0.15%)	⬆️

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