feat: add n8n workflow automation integration (fixes MervinPraison/PraisonAI#1396)

- Add N8nWorkflowTool class for executing n8n workflows
- Add n8n_workflow and n8n_list_workflows decorator functions
- Provides access to 400+ integrations (Slack, Gmail, Notion, databases, APIs)
- Includes comprehensive unit tests with mocking
- Add example usage and documentation
- Optional httpx dependency with lazy loading
- Bidirectional PraisonAI ↔ n8n integration support

Author: MervinPraison