.NET: Workflow observability (#959)
* Add basic Workflow telemetry
* Add sample
* Add source propagation for executor spans
* Fix tests and address comments
* Fix formatting
* Fix declarative unit tests
* Address comments
* Remove Microsoft.Extensions.AI.Agents.EnableTelemetry
* Formatting
* Formatting
* Formatting
* fix solution
* Address comments
* Add workflow json definition for serialization
* Formmating
* Address comments
Tao Chen