Remove hardcoded assistants version

[danai-antoniou]

Describe the change
OpenAI released assistants v2, however the code currently hardcodes v1 in every assistant and thread method.

Provide OpenAI documentation link
https://platform.openai.com/docs/assistants/migration

Describe your solution
I've added an AssistantVersion parameter in the client config, which defaults to v1, so there will be no breaking change for anyone.
Users will be able to use v2 and any future versions by passing the version in the client config.

Tests
I've been using this in our fork of go-openai.

Issue: #717

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.67%. Comparing base (774fc9d) to head (4c2727d).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #719      +/-   ##
==========================================
+ Coverage   98.46%   98.67%   +0.21%     
==========================================
  Files          24       24              
  Lines        1364     1131     -233     
==========================================
- Hits         1343     1116     -227     
+ Misses         15        9       -6     
  Partials        6        6              

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[sashabaranov]

Thank you for the PR!