perf: reusing the inner httpx client

[frascuchon]

This PR introduces the changes to the API client to reuse the internal httpx client for several requests. These changes could help to mitigate some reported errors related to this issue encode/httpx#1027

Also, stop creating new threads after the utils.py::setup_loop_in_thread is called.

Closes #1646
Refs: The Slack thread

[codecov]

Codecov Report

Merging #1640 (5c7b3da) into master (843f06d) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1640      +/-   ##
==========================================
- Coverage   95.59%   95.52%   -0.08%     
==========================================
  Files         133      133              
  Lines        6339     6346       +7     
==========================================
+ Hits         6060     6062       +2     
- Misses        279      284       +5     

Flag	Coverage Δ
pytest	95.52% <100.00%> (-0.08%)	⬇️

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

Impacted Files	Coverage Δ
src/rubrix/client/api.py	95.98% <100.00%> (-0.02%)	⬇️
src/rubrix/client/sdk/client.py	95.23% <100.00%> (+0.41%)	⬆️
src/rubrix/client/sdk/text2text/api.py	100.00% <100.00%> (ø)
src/rubrix/client/sdk/text_classification/api.py	100.00% <100.00%> (ø)
src/rubrix/client/sdk/token_classification/api.py	100.00% <100.00%> (ø)
src/rubrix/client/sdk/users/api.py	100.00% <100.00%> (ø)
src/rubrix/utils.py	100.00% <100.00%> (ø)
src/rubrix/client/sdk/commons/errors_handler.py	89.65% <0.00%> (-6.90%)	⬇️
...ubrix/labeling/text_classification/label_errors.py	86.74% <0.00%> (-3.62%)	⬇️

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