[WORKFLOW SDK FEATURE REQUEST] Implement "new_guid" on WorkflowContext

[olitomlinson]

The ability to generate a replay-safe GUID is missing from the API surface of WorkflowContext.

This is a fundamental feature of all Workflow SDKs. e.g. https://github.com/dapr/dotnet-sdk/blob/9a16009d98738fd4f11d1cd7ff58e5afc4fb945a/src/Dapr.Workflow/Worker/Internal/WorkflowOrchestrationContext.cs#L519