Add Workflow Executor node processor for triggering child workflows

Summary

Add a Workflow Executor node processor that enables executing another FlowDrop workflow from within a workflow. This supports multiple execution modes for different use cases: synchronous execution with result waiting, asynchronous execution with deferred result retrieval, and fire-and-forget for background processing.

Business Value

• Enables modular workflow design by allowing reusable sub-workflows
• Supports complex orchestration patterns like fan-out/fan-in
• Reduces workflow complexity by breaking large workflows into smaller, maintainable pieces
• Provides flexibility in execution timing based on use case requirements

Execution Modes

• Job (Synchronous): Execute the child workflow and wait for completion before continuing. Best for sequential processing where results are needed immediately.
• Async: Execute the child workflow and return a session ID immediately. Use Workflow Results Retriever to get results later. Best for parallel execution patterns.
• Fire & Forget: Queue the workflow for background execution with no result tracking. Best for spawning independent tasks that don't affect the parent workflow.

Parameters

• workflow_id: Select the workflow to execute (dropdown of available workflows)
• input_data: Object data to pass to the child workflow (connectable input port)
• execution_mode: Choose between job, job_async, or job_fire_forget
• timeout: Maximum wait time in seconds (job mode only)
• poll_interval_ms: How often to check for completion (job mode only)

Outputs

• success: Boolean indicating execution/start success
• status: Execution status (completed, running, pending, failed, timeout)
• session_id: Session ID for async result retrieval
• outputs: Workflow outputs when completed
• execution_time_ms: Total execution time

Safety Features

• Circular workflow reference detection prevents infinite loops
• Maximum execution depth (5 levels) prevents runaway recursion
• Parent context propagation tracks the execution chain