Provide ids of documents in the retriever API

[lebeg]

I'm using elasticsearch as my vector store, but I think for any store there is a concept of an id. Each Document has an id assigned to it when the document is stored. When I'm using the retriever API, such as get_relevant_documents I get a list of documents, but not their ids. There is an API for having an external docstore for handling the mappings of documents to their ids in the MultiVectorRetriever, but I can't currently understand why its needed and why ids of the vector store can not be used directly.