Vector Store support

[kamillitman]

This PR introduces support for OpenAI Vector Store with Assistant struct.

Main changes:

• [ ] New assistants::openai::openai_vector_store module with a new public struct OpenAIVectorStore.
• [ ] Basic methods for OpenAIVectorStore including initialization, status check and deletion. We may need to add additional methods in the future. For this PR I kept it to the basics.
• [ ] Modification to OpenAIAssistant to support attaching a Vector Store and running file search against it.
• [ ] Modification to use_openai_assistnat example to showcase new capabilities.

Example usage:

// You still need to first upload a file
let openai_file = OpenAIFile::new(&file_name, bytes, &api_key, true).await?;

// Create a Vector Store and assign the file to it
    let openai_vector_store = OpenAIVectorStore::new(None, "Concerts", &api_key)
        .debug()
        .upload(&[openai_file.id.clone()])
        .await?;

// Attach Vectore Store to an Assistant and use it for search
    let concert_info = OpenAIAssistant::new(OpenAIModels::Gpt4o, &api_key, true)
        .await?
        .version(OpenAIAssistantVersion::V2)
        .vector_store(openai_vector_store.clone())
        .await?
        .get_answer::<ConcertInfo>(
            "Extract the information requested in the response type from the attached concert information.
            The response should include the genre of the music the 'band' represents.
            The mapping of bands to genres was provided in 'bands_genres' list in a previous message.",
            &[], // No files attached to the message. Assistant will use the Vector Store
        )
        .await?;

    // Delete the Vector Store
    openai_vector_store.delete().await?;