.NET: [Feature]: Allow enabling workflow level Open Telemetry when using AgentWorkflowBuilder

### Description

Unless I (and copilot) are missing something when using `AgentWorkflowBuilder` to create a workflow, there is no way to enable OpenTelemetery at the workflow level (the agents and/or chatclient can have it enabled). It would be useful to be able to see the workflow telemetry when debugging. I think there are reasons why you're not allowing any middleware to be added to the workflow builder when using `AgentWorkflowBuilder`, so maybe just a parameter to the existing methods to allow enabling it.

### Code Sample

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### Language/SDK

.NET