Draft: Add gitlab_integration_datadog resource
Add support for configuring Datadog integration on GitLab projects via Terraform.
Features:
- Full CRUD operations for Datadog project integration
- Support for API key, URL, environment, service, site configuration
- Custom tags support with key:value format
- Archive trace events option for job log collection
- Import/export state management
- Follows existing integration resource patterns
The resource enables automation of Datadog onboarding for GitLab projects, supporting both datadoghq.com and datadoghq.eu sites with configurable environment tags and custom metadata.
Closes #6101