Updating the index when using lazy embeddings

[jamesbraza]

https://github.com/Future-House/paper-qa/pull/526 added the ability to defer embeddings when we don't need them. A natural extension of this is to speed up index builds by:

1. Deferring embedding creation from index build time to index usage time (inference time)
2. At index usage (e.g. paper_search), when we create the embeddings, we update the index to include the new embeddings

The main benefit is going from 0-1 faster when trying out different indexes from scratch