Implement vector store search, retrieve file content operations

[cfraz89]

• Implement vector store search as in
  • File Search guide
  • Search api
  • OpenAPI spec

A basic query looks like this:

    let results = client
        .vector_stores()
        .search(
            &vector_store.id,
            VectorStoreSearchRequest {
                query: "uber profit".into(),
                ..Default::default()
            },
        )
        .await?;

• Also implement retrieve file contents api
• Add attributes property to CreateVectorStoreFileRequest
• Add an example for vector store search.

[64bit]

Thank you for the PR, really quality work - from matching spec to adding example!

I committed a few suggestions to cleanup after example is run - delete vector store and files to avoid any costs.