feat: support for Assistant stream mode and implemented stream event callbacks

[CallOrRet]

This update enhances the existing functionality by adding support for Assistant stream mode and stream event callbacks without introducing any breaking changes or modifications to the existing API endpoints.

The implemented features are in line with the OpenAI Assistant streaming API, as documented here: OpenAI Assistant Streaming API

[codecov]

Codecov Report

Attention: Patch coverage is 4.47761% with 64 lines in your changes are missing coverage. Please review.

Project coverage is 93.42%. Comparing base (774fc9d) to head (abb0e96).
Report is 4 commits behind head on master.

❗ Current head abb0e96 differs from pull request most recent head 12cdb6d. Consider uploading reports for the commit 12cdb6d to get more accurate results

Files	Patch %	Lines
assistant_stream.go	0.00%	36 Missing ⚠️
stream_reader.go	9.67%	28 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #698      +/-   ##
==========================================
- Coverage   98.46%   93.42%   -5.04%     
==========================================
  Files          24       25       +1     
  Lines        1364     1201     -163     
==========================================
- Hits         1343     1122     -221     
- Misses         15       73      +58     
  Partials        6        6              

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[sashabaranov]

@push-edp thank you for the PR! Could you please add tests so coverage check would pass?

[CallOrRet]

@push-edp thank you for the PR! Could you please add tests so coverage check would pass?

Sure, I'll add the tests shortly.