Adds a Nextjs client

[acmeyer]

This pull request adds a simple Nextjs frontend client to the plugin for easier management of uploading files. There's also a test area where you can write a query and see what documents would be retrieved.

The Nextjs app borrows heavily from the openai-cookbook repo with some slightly modifications and improvements.

Other slight server changes are adding CORS and updating Pinecone implementation to accept namespace.