Add support for expires_after field in CreateFileRequest and expires_at field in OpenAIFile

[MakotoE]

• The expires_after: Option<FileExpiresAfter> field is added to CreateFileRequest. This new field is added to the post https://api.openai.com/v1/files request when defined.
  • https://platform.openai.com/docs/api-reference/files
  • I tried to follow the patterns of other fields by adding the FileExpiresAfterAnchor enum, even though it only contains one option, CreatedAt.
• The expires_at: Option<u32> field is added to OpenAIFile.
  • https://platform.openai.com/docs/api-reference/files/object
  • I found that expires_at can be null if you didn't specify expires_after in the request, even though the OpenAI documentation does not describe it as being optional. That is why I made the type Option<u32>, not u32.

[64bit]

Thank you for spec update!