* [BREAKING] refactor: Decouple Checkpointing and Execution APIs
With this change, Checkpointing becomes an property of an IWorkflowExecutionEnvironment. This lets environments that are tightly-coupled to their CheckpointManager avoid needing to present APIs that would not work (e.g. taking in an InMemory CheckpointManager for Durable Tasks, for example)
* refactor: Normalize IsCheckpointingEnabled naming
Track the last CheckpointInfo in InProcessRunner so that newly created
checkpoints reference their parent. When resuming from a checkpoint,
the resumed-from checkpoint becomes the parent of the next checkpoint.
Adds tests verifying:
- First checkpoint has null parent
- Subsequent checkpoints chain parents correctly
- Checkpoint after resume references the resumed-from checkpoint