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 95.01%. Comparing base (00ce055) to head (4b5a50e).
⚠️ Report is 1 commits behind head on master.

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

Additional details and impacted files

@@ Coverage Diff @@## master #1004 +/- ##
==========================================
+ Coverage 94.96% 95.01% +0.05% 
==========================================
Files 58 59 +1 Lines 5438 5501 +63 ==========================================
+ Hits 5164 5227 +63 
Misses 274 274 

Flag	Coverage Δ
integration	92.07% <90.62%> (+0.47%)	⬆️
unit	85.96% <98.43%> (+0.14%)	⬆️

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