Use async source storage clients

[smokestacklightnin]

Feature description

Even with native Ragna machinery like as_awaitable, that machinery wraps synchronous tools, for example source storage clients, we will not get the full benefit of async. Particularly, if we want to fetch data/metadata from a source storage in parallel, we necessarily must use an async client (assuming we use a client instead of http requests).

I have checked, and all three currently supported source storages, Chroma, Lance, and Qdrant have async clients, so it is just a matter of making the migration.

Value and/or benefit

No response

Anything else?

This is part of the solution to #557