Make DaService::get_extraction_proof an async function

[preston-evans98]

Is your feature request related to a problem? Please describe.
"My get_extraction_proof is async and requires communication using the client. I solve this by just getting the extraction proof in get_finalized_at unconditionally. But I guess that makes some unnecessary work for pure full node mode." This feature request originated in Discord.

Describe the solution you'd like
Make DaService::get_extraction_proof an async function

Additional context
My (Preston's) response in Discord: "We can definitely make get_extraction_proof async as well. I had initially envisioned it as a mathematical operation - but it makes sense that it would be baked into the remote client."