Add SAST Config UI placeholder page

What does this MR do?

This adds a basic blank page entry point for #225224 (closed).

The new controller uses the same permissions as the Security Configuration controller, as that page will be how users reach this one (see #220573 (closed)).

Feature Flag

Enable the sast_configuration_ui feature (development) flag to test this locally.

Screenshots

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