Update for Prefect 3 rc

[jlowin]

Updates CF for:

• run_coro_as_sync has a more specific behavior; for the moment i have removed it and explicitly created sync/async routes. We can recombine later.
• default result type is str
• use graph to detect upstream dependencies to avoid infinite loop
• agents create completions instead of controllers, which means that agents can become pluggable interfaces for custom attributes / libraries / memories, etc.