Converting add_texts_and_embeddings to async

[ThomasRochefortB]

This PR converts the VectorStore.add_texts_and_embeddings functions to async as discussed in #776

[jamesbraza]

Thanks for doing this @ThomasRochefortB, I triggered CI for this. Please forgive us for the bad contributor experience at the moment

• Sadly it seems GitHub Action secrets don't propagate to fork's CI
• Fixed type problems from llmclient #770 is going to fix the mypy errors

That being said, locally testing your code I still get this error:

paperqa/docs.py: note: In member "_build_texts_index" of class "Docs":
paperqa/docs.py:519:9: error: Value of type "Coroutine[Any, Any, None]" must be
used  [unused-coroutine]
            self.texts_index.add_texts_and_embeddings(texts)
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
paperqa/docs.py:519:9: note: Are you missing an await?

Can you add an await there in Docs._build_texts_index?