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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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