Activity and sub-orchestrator cancellation support

[cgillum]

The TaskOptions class provided when scheduling activities and sub-orchestration calls supports providing a CancellationToken. However, it's not yet wired up to actually cancel the task. This issue tracks adding support for cancellation.

• Implement the feature for activity calls, sub-orchestrator calls, and retry handlers.
• Add detailed <remarks> documentation with examples for how cancellation can be used.
• Add tests that cover all the scenarios supported by task cancellation