Add Workflow Results Retriever node for async workflow result collection

Summary

Add a Workflow Results Retriever node processor that retrieves results from asynchronously executed workflows. This complements the Workflow Executor node's async mode by providing a way to collect results when they're ready.

Business Value

• Enables parallel workflow execution patterns
• Supports long-running child workflows without blocking the parent
• Provides flexible result collection with wait or poll options
• Enables building responsive workflows that spawn background tasks

Parameters

• session_id: The session ID from a Workflow Executor node (async mode)
• wait_for_completion: Block until the workflow completes or times out
• timeout: Maximum time to wait for completion (1-3600 seconds)
• poll_interval_ms: How often to check for completion (100-30000ms)

Outputs

• success: Boolean indicating if results were retrieved successfully
• status: Current status (completed, running, pending, failed, timeout)
• outputs: The outputs from the executed workflow
• error_message: Error details if retrieval failed
• wait_time_ms: How long we waited for results

Use Cases

• Collect results from multiple parallel workflow executions
• Check status of long-running background workflows
• Implement timeout handling for slow workflows
• Build dashboard workflows that monitor child workflow status