feat: Add n8n workflow automation integration (#19)

* 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

* fix: resolve critical n8n integration issues

- Export n8n_list_workflows from __init__.py (fixes ImportError)
- Fix module path in tools registry to use absolute imports
- Update __getattr__ to handle absolute module paths
- Fix timeout parameter not being used in _wait_for_execution
- Update test mocking to work with lazy imports

Addresses P0 and P1 issues identified in code review.

Co-authored-by: Mervin Praison <MervinPraison@users.noreply.github.com>

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Co-authored-by: MervinPraison <454862+MervinPraison@users.noreply.github.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Mervin Praison <MervinPraison@users.noreply.github.com>

Author: 3 people