fix: offload async request body compression to a worker thread

[vdusek]

The async ImpitHttpClientAsync.call() serialized and compressed request bodies inline on the event loop. Compression is CPU-bound (and brotli is becoming the default via the Apify SDK), so it blocked the loop for the whole duration of the compression, stalling every other concurrent task (response reads, the platform events websocket, other in-flight requests).

Request preparation is now offloaded to a worker thread via asyncio.to_thread whenever there is a body to compress. Bodyless requests (the common polling, listing, and get path) stay inline to avoid a needless thread-dispatch hop. The synchronous client is unchanged, as it has no event loop to block.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.60%. Comparing base (a8a393b) to head (5e7f697).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #950      +/-   ##
==========================================
- Coverage   94.61%   94.60%   -0.02%     
==========================================
  Files          58       58              
  Lines        5239     5241       +2     
==========================================
+ Hits         4957     4958       +1     
- Misses        282      283       +1     

Flag	Coverage Δ
integration	92.38% <100.00%> (-0.29%)	⬇️
unit	83.74% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.