add from to add texts from embeddings

[shibuiwilliam]

Why

• Since making vectors of texts can be done outside of langchain Faiss, this PR is to add functionality to pass text and its vector pair to initialize and add embedding to Faiss.

What

• Add from_embedding method in Faiss to initialize Faiss index by passing the embeddings paired with original text made outside of the langchain.
• Add add_embedding method to add embedding paired with original text to append the embedding made outside of the langchain.