[WORKFLOW SDK FEATURE REQUEST] Make `WorkflowRuntime` async native

[seherv]

The worker runs activities on a background thread while the application runs on its own loop. This has caused attached to a different loop errors on async activities that use async grpc clients. There are patches for lazy initialization of grpc clients so they start on the caller's loop, but they are not ideal.

Depends on #1089

---

Acceptance criteria:

• async with WorkflowRuntime() and await wfr.run() run on the caller's loop
• Async activities execute on the caller's loop
• No resources are leaked
• Sync start() / shutdown() still works

[seherv]

/assign