Add vector store endpoints

[willywg]

Hi...

I'm excited to submit this PR for the issue #463: "Add the Vector Store, Vector Store Files, and Vector Store Batch Files APIs." With the recent release of OpenAI's Assistant V2, new endpoints have been introduced, including VectorStores, VectorStoreFiles, and VectorStoreBatchFiles. These endpoints are essential for leveraging the enhanced capabilities of the updated Assistant.

In this PR, I have implemented these new endpoints, ensuring compatibility and extending our API's functionality. Additionally, I have updated the README.md with usage examples to facilitate a better understanding for developers integrating these services.

Please find below the summary of changes:

• Added new API endpoints for VectorStores, VectorStoreFiles, and VectorStoreBatchFiles.
• Updated the README.md with clear and concise examples for each of the new endpoints.
• Ensured that all changes adhere to the existing code standards and practices.

Best regards.

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• [x] Have you followed the guidelines in our Contributing document?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
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[bvsatyaram]

I have been working on this branch for a couple of days. It is stable and works as expected. I think it is good to go.

@alexrudall - When can we get this merged?

[sumitngupta]

Thank you for the work on this PR! Getting our setup updated to utilize it.

[alexrudall]

Released in v7.1

[alexrudall]

Thanks for this excellent PR @willywg !