[Compute Separation] Make ConnectedWorkerChannel stream timeout configurable

[brettsam]

Context

ConnectedWorkerChannel.StartWorkerProcessAsync currently hardcodes a 30-second timeout for BeginInboundProcessing:

csharp BeginInboundProcessing(startStreamTimeout: TimeSpan.FromSeconds(30));

In the existing GrpcWorkerChannel, this value comes from WorkerConfig.CountOptions.ProcessStartupTimeout, which is customer-configurable via worker config.

Problem

In the separated compute model, there is no worker.config available on the host side since the worker runs on separate compute. We need a different configuration mechanism for this timeout.

Considerations

• This may need to be a platform-level configuration knob rather than a customer-facing setting, since the host no longer has access to customer payload configuration.
• Consider whether ExternalWorkerOptions (M4) is the right place for this, or if it belongs in hosting config.

References

• PR feedback: [Compute Separation] M2: ConnectedWorkerChannel + channel manager + invocation dispatcher #11655 (comment)
• Parent epic: [Epic] Compute Separation — Enable external workers on separate compute #11640