feat!: Make HTTP client pluggable with abstract base classes

[vdusek]

Summary

• Introduce HttpClient and HttpClientAsync ABCs that users can extend to provide custom HTTP client implementations via ApifyClient(http_client=...) and ApifyClientAsync(http_client=...)
• Factor impit-specific logic into ImpitHttpClient/ImpitHttpClientAsync, keeping _base.py free of any impit dependency
• Add HttpResponse protocol with full streaming support (iter_bytes, aiter_bytes, read, aread, close, aclose)
• Export all public types (HttpClient, HttpClientAsync, HttpResponse, ImpitHttpClient, ImpitHttpClientAsync) from apify_client

Issue

• Closes: Allow to pass own http_client to ApifyClient and ApifyClientAsync #416

Test plan

• CI passes

[codecov]

Codecov Report

❌ Patch coverage is 96.77419% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.67%. Comparing base (7895a4e) to head (f5ab55e).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/apify_client/_http_clients/_base.py	92.85%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #641       +/-   ##
===========================================
+ Coverage   75.60%   96.67%   +21.07%     
===========================================
  Files          45       45               
  Lines        4271     4306       +35     
===========================================
+ Hits         3229     4163      +934     
+ Misses       1042      143      -899     

Flag	Coverage Δ
integration	94.70% <85.48%> (?)
unit	75.94% <96.77%> (+0.33%)	⬆️

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