Create feature flag for threat monitoring alerts tab

What does this MR do?

Create feature flag for threat monitoring alerts tab

• show empty component
• add tests

Screenshots (strongly suggested)

With feature flag disable	With feature flag enabled

How to reproduce

1. Pull down code and ensure you have an Ultimate license uploaded
2. Navigate to a project with alerts and navigate to Security & Compliance => Threat Monitoring
3. Enable the feature flag with rails c && Feature.enable(:threat_monitoring_alerts)
4. Reload the page

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #287681 (closed) Related to #287675 (closed)