Creates new on-demand scans routes

What does this MR do?

This creates two new routes to access the On-demand scans form: new and edit. These new routes will be used as part of the upcoming Saved scans feature.

This also creates the dast_saved_scans feature flag.

When the feature flag is enabled, the Security & Compliance > On-demand scans nav item points to the new route.

In a follow-up issue, new fields will be added to the On-demand scans form (and hidden behind the feature flag).

The edit route will remain inaccessible via the navigation items until the edit mode is implemented (#295242 (closed)).

How to test this?

1. The current route should still be accessible at /:namespace/:project/-/on_demand_scans
2. Enable the feature flag:

echo "Feature.enable(:dast_saved_scans)" | rails c

3. Navigate to the new route at /:namespace/:project/-/on_demand_scans/new.
4. Navigate to the edit route at /:namespace/:project/-/on_demand_scans/:id/edit, where :id can be anything (validation will be added in #295242 (closed)).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry (behind a feature flag)
• [-] 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

Related to #295240 (closed)