feat: Add Webhook Notification Plugin

[manavgup]

Summary

Adds a comprehensive webhook notification system for MCP Gateway events with support for multiple authentication methods, retry logic, and customizable payload templates.

Features Added

• Webhook Notification Plugin (plugins/webhook_notification/)
  • HTTP webhook notifications for events, violations, and state changes
  • Multiple authentication methods (Bearer token, API key, HMAC signatures)
  • Retry logic with exponential backoff
  • Customizable JSON payload templates per event type
  • Event filtering and concurrent webhook delivery
  • Comprehensive error handling and timeout support

Event Types Supported

• Tool execution (success/error)
• PII detection violations
• Rate limit violations
• Resource fetch operations
• Prompt fetch operations
• Plugin errors and violations

Files Added/Modified

• plugins/webhook_notification/webhook_notification.py - Main plugin implementation
• plugins/webhook_notification/README.md - Comprehensive documentation
• plugins/webhook_notification/TESTING.md - Testing procedures and examples
• plugins/webhook_notification/test_config.yaml - Example test configuration
• plugins/webhook_notification/plugin-manifest.yaml - Plugin metadata
• tests/unit/mcpgateway/plugins/plugins/webhook_notification/ - Complete test suite
• plugins/config.yaml - Updated with webhook plugin configuration

Configuration Example

- name: "WebhookNotification"
  kind: "plugins.webhook_notification.webhook_notification.WebhookNotificationPlugin"
  hooks: ["tool_post_invoke", "prompt_post_fetch"]
  mode: "permissive"
  priority: 900
  config:
    webhooks:
      - url: "https://hooks.slack.com/services/YOUR/WEBHOOK"
        events: ["violation", "tool_success", "rate_limit_exceeded"]
        authentication:
          type: "bearer"
          token: "${env.SLACK_WEBHOOK_TOKEN}"
        retry_attempts: 3

Testing

• Comprehensive unit test coverage (authentication, retry logic, event filtering)
• Integration tests with PluginManager
• Manual testing procedures with webhook.site integration
• Performance and load testing examples

Documentation

• Complete README with usage examples and configuration options
• Detailed testing guide with multiple testing approaches
• Security considerations and best practices
• Troubleshooting section

Test Plan

• Unit tests for all authentication methods
• Integration tests with plugin manager
• Retry logic and error handling tests
• Template rendering and customization tests
• Event filtering and webhook delivery tests
• Manual testing with webhook.site