[WORKFLOW SDK FEATURE REQUEST] Use `grpc.aio` in the durabletask worker

[seherv]

TaskHubGrpcWorker consumes the sidecar stream on a dedicated StreamReader thread and pushes things to the event loop through a queue. We already have grpc.aio in the SDK, we can consume it asynchronously and remove the thread and the queue.

This should not change the public API.

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Acceptance criteria:

• The worker consumes the stream asynchronously
• WorkflowRuntime.start() / .shutdown() still works
• Reconnect on stream drop and on transient errors still works
• No leaked resources on shutdown

[seherv]

/assign