Fix update_embeddings() for FAISSDocumentStore

[oryx1729]

There are three main use cases for update_embeddings():

1. Creating embeddings in a new document store after writing the documents. In this case, embeddings are created for all the documents in the document store. The default parameters for update_embeddings() are sufficient for this case.

2. Incremental update after adding more documents in an existing index. In this case, update_embeddings() can be called with update_existing_embeddings set to False to prevent recomputation of existing embeddings.

3. Create/Update embeddings for a subset of documents in a document store using the filters parameter. For FAISSDocumentStore, filters cannot be used in conjunction with the update_existing_embeddings set to True.

This PR resolves #885 where update_embeddings() could result in more embeddings than the number of documents and adds a new method get_embedding_count() for all document stores.

[tholor]

Looking good. Only minor comment. Have you verified that update_embeddings() is working correctly for milvus?

[oryx1729]

Looking good. Only minor comment. Have you verified that update_embeddings() is working correctly for milvus?

Yes, it works correctly for Milvus as unlike FAISS, we can delete vectors using their IDs from the index.