Add "Show last" Threat Monitoring filter

What does this MR do?

This adds the "Show last" filter dropdown to the Threat Monitoring UI, allowing for statistics over different time ranges to be displayed.

Part of WAF statistics reporting.

Screenshots

As the backend isn't in place yet, these were created by applying this patch to mock the endpoint responses, and also enabling the threat_monitoring feature flag.

Before	After
n/a

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