Add Index Management Features

[mcheemaa]

We would like to add index management features to the ruby-openai-pinecone gem to make it easier for users to create, delete, list, describe, and configure Pinecone vector indices.

Here's a list of the desired features:

1. List all indices
2. Describe an index
3. Create an index
4. Delete an index
5. Configure an index

These features should include a new Index class with methods for each operation. The class should handle requests to the Pinecone API with proper headers and configuration.

If you're interested in contributing, please feel free to submit a pull request.