Add webhook integration

[Djiit]

This issue is to add a generic webhook integration to gong.

Overview

The webhook integration should allow sending HTTP POST notifications to custom webhook endpoints about pending PR reviews.

Implementation Details

1. Create a new package in internal/integrations/webhook
2. Implement the webhook integration interface
3. Add support for configurable webhook URLs, headers, and payload templates
4. Add configuration parameters for webhook notifications

Acceptance Criteria

• Webhook notifications can be configured globally or per rule
• Support for custom headers and payload templates
• Webhook integration can be used alongside other integrations (Slack, Comment, etc.)
• The integration is properly documented in the docs
• Unit tests are written for the implementation
• Update examples in configuration documentation

References

See existing integrations like Slack integration for a good example to follow.

Labels: integration, help wanted