Add File purposes as constants

[revett]

Describe the change

• When uploading a File, you need to define a .Purpose
• Currently this is a string that is passed to the FileRequest type
• It would be nicer to have these as constants

See: https://platform.openai.com/docs/api-reference/files

Describe your solution

Add a constant for each supported purpose as defined in the OpenAI API.

Tests

No tests needed.

Additional context

Not related to an issue, just something I saw when using the new Assistants API.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1846572) 98.38% compared to head (aa3560d) 98.38%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #577   +/-   ##
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  Coverage   98.38%   98.38%           
=======================================
  Files          24       24           
  Lines        1300     1300           
=======================================
  Hits         1279     1279           
  Misses         15       15           
  Partials        6        6           

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[floodfx]

I added something similar in #568 but with more type safety. We should probably merge these efforts? I can add the additional types to #568. I am unclear whether the type safe approach I used is typical of this project though?

[revett]

I added something similar in #568 but with more type safety. We should probably merge these efforts? I can add the additional types to #568. I am unclear whether the type safe approach I used is typical of this project though?

Thanks! I've rebased your merged changes in to this PR. Looks like we still need two more purposes added.

@sashabaranov This is ready for review 👍

[floodfx]

Looks great to me!

[sashabaranov]

Thank you!