Streaming support

[phact]

This pull request introduces support for streaming workflows, adds new functionality for managing debug sessions, and improves the handling of execution flows in the StepFlow system. Key changes include adding a Streaming variant to FlowResult, enhancing debug session management with tokio::sync::Mutex, and updating the ValueResolver to handle streaming results.

Streaming Workflow Support:

• Added a Streaming variant to the FlowResult enum to represent streaming data with associated metadata, chunk information, and a final chunk indicator (crates/stepflow-core/src/flow_result.rs).
• Updated the ValueResolver to pass through streaming results without applying skip actions and handle streaming results in objects and arrays (crates/stepflow-execution/src/value_resolver.rs). [1] [2] [3]
• Modified the EvalComponent to serialize streaming results into JSON format for external use (crates/stepflow-builtins/src/eval.rs).

Debug Session Management:

• Replaced WorkflowExecutor in debug_sessions with an Arc<tokio::sync::Mutex<WorkflowExecutor>> to allow safe concurrent access and reuse of debug sessions (crates/stepflow-execution/src/executor.rs). [1] [2]
• Added methods to retrieve existing debug sessions, clean up sessions after execution, and manage workflow executors for streaming (crates/stepflow-execution/src/executor.rs). [1] [2] [3]

Execution Flow Enhancements:

• Introduced cleanup mechanisms for pending executions and debug sessions to prevent memory leaks (crates/stepflow-execution/src/executor.rs).
• Created a wrapper to implement the Executor trait for StepFlowExecutor, enabling integration with the plugin system (crates/stepflow-execution/src/executor.rs).

Additional Improvements:

• Added a Clone implementation for DependencyTracker and other structs to support cloning where needed (crates/stepflow-analysis/src/tracker.rs, crates/stepflow-core/src/workflow/step.rs, crates/stepflow-execution/src/value_resolver.rs). [1] [2] [3]
• Introduced a streaming flag in the Step struct to indicate whether a step is a streaming step (crates/stepflow-core/src/workflow/step.rs). [1] [2] [3]