Sub-orchestration and Terminate SDK support

[cgillum]

A few major additions in this PR are:

1. Sub-orchestration APIs for orchestrations written in Go
2. Support for terminating orchestrations written in Go
3. Support for cascade termination (which requires some changes in the protobuf definitions)
4. Breaking changes related to improvements in the options pattern for some APIs

These are largely some changes that we want to have in place in preparation for building a Go SDK for Dapr Workflow, after which it's going to be harder to make breaking changes.

Breaking changes should only impact direct users of durabletask-go. Indirect users (e.g., via Dapr Workflow) won't be impacted by these breaking changes.

Validation:

• Unit and integration tests passing
• Language SDK tests passing (e.g. .NET, Java, and Python)