Support Attachments in MessageRequest

[kodernubie]

Describe the change
Add Attachments field in MessageRequest struct so we can attach file to thread message and can refer the file in chat content.

Provide OpenAI documentation link
https://platform.openai.com/docs/api-reference/messages/createMessage

Describe your solution
Adding Attachments Field and tools type const in message.go

Tests
using unit test and example app

Additional context

Issue: -

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.86%. Comparing base (774fc9d) to head (b130e7e).
Report is 70 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #890      +/-   ##
==========================================
+ 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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[sashabaranov]

Thank you!