feature/add-callback-and-async-callback

[sebastianbuechler]

This PR tackles #153 and #156 as they are related.

Addtionally to the reply method of the RequestHandler, I added a replyCallback and replyCallbackAsync that will take only the defined callback structures.

The replyCallback was straight forward to implement, where the replyCallbackAsync needed so more adjustments for the async handling throug out the codebase.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03% 🎉

Comparison is base (26eb57d) 99.25% compared to head (9a00440) 99.28%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #157      +/-   ##
==========================================
+ Coverage   99.25%   99.28%   +0.03%     
==========================================
  Files          24       24              
  Lines         267      280      +13     
==========================================
+ Hits          265      278      +13     
  Misses          2        2              

Files Changed	Coverage Δ
lib/src/adapters/dio_adapter.dart	100.00% <100.00%> (ø)
lib/src/handlers/request_handler.dart	100.00% <100.00%> (ø)
lib/src/interceptors/dio_interceptor.dart	100.00% <100.00%> (ø)

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