Store index ids outside of configuration when content is disabled

[davidmezzetti]

When a new embeddings instance is created, the default settings do not store content. In this mode, index ids are added to a separate array and stored directly in the embeddings configuration.

#652 is changing the default configuration format from pickle to json. Given this change, it's best to store ids in a file separate from configuration.

This change will support reading existing embeddings indexes with config["ids"] set. All save calls moving forward will store ids in a separate file, even for existing indexes.

This change has no impacts to indexes with content storage enabled.