feat: Add HTTPX-based HTTP client

[vdusek]

Adds HttpxHttpClient / HttpxHttpClientAsync as built-in alternatives to the default Impit transport, behind an optional extra: pip install "apify-client[httpx]". Impit stays the default.

Stacked on #1022, whose shared request pipeline the adapters plug into: they implement only the transport hooks (send_request, is_retryable_transport_error, is_timeout_error, close() / aclose()) and apply the same retry policy as Impit — every error from the library's own hierarchy is transient except the permanently-failing types. A response event hook keeps HTTPX's shared cookie jar from leaking server cookies into later API requests. Unit tests run the built-in client matrix (Impit + HTTPX) through the parametrized fixtures.

Stack: #1022 (pipeline) → #1004 (this) → #1013 (docs).

✍️ Drafted by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 98.43750% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.97%. Comparing base (35309d5) to head (2760925).

Files with missing lines	Patch %	Lines
src/apify_client/http_clients/__init__.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@                       Coverage Diff                       @@
##           unify-http-request-pipeline    #1004      +/-   ##
===============================================================
+ Coverage                        94.93%   94.97%   +0.04%     
===============================================================
  Files                               58       59       +1     
  Lines                             5406     5469      +63     
===============================================================
+ Hits                              5132     5194      +62     
- Misses                             274      275       +1     

Flag	Coverage Δ
integration	91.97% <90.62%> (+0.44%)	⬆️
unit	85.44% <98.43%> (+0.15%)	⬆️

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