added example of using assistant api

[himasyuri]

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I added example of using assistant api: creating simple assistant and how to use it.

I use this doc: https://platform.openai.com/docs/api-reference/assistants

My changes can help new users create their assistant using openai assistant API. It is simple example of using. This will help save them time. When I create this example, I used articles and API doc from openai site(https://platform.openai.com/docs/api-reference/assistants). But in articles assistant's code wrote on python and I adapted this code for golang using this library.

I test this code on my local machine. And I use the same code for my telegram bot and on production on my work.

If this pull request needs be edited, I will do this.

Issue: not created issue, must i create my own issue?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.86%. Comparing base (774fc9d) to head (39470e4).
Report is 68 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #888      +/-   ##
==========================================
+ Coverage   98.46%   98.86%   +0.40%     
==========================================
  Files          24       26       +2     
  Lines        1364     1761     +397     
==========================================
+ Hits         1343     1741     +398     
+ Misses         15       14       -1     
  Partials        6        6              

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