Add persistence to `LocalVectorStoreDriver`

[vasinov]

Many QA, extraction, and summarization use cases don't require a sophisticated vector store to function correctly. Today, LocalVectorStoreDriver works great for those use cases in the context of conversational "in the moment" workflows but lacks a way to persist embeddings on disk.

I propose adding an on-by-default setting for persisting and automatically loading LocalVectorStoreDriver.entries from the disk with an optional custom path to the file.

[derekpai]

#109