Implement DAST variables update workflow

What does this MR do and why?

Implement DAST variables update workflow as described below:

1. When users select the Edit dropdown, the DAST variables modal should be opened
2. The DAST variable values should be populated with the variable values
3. Update the copy for the modal header and primary button
4. Add a delete button

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After
Screen_Recording_2024-12-26_at_12.25.07	Screen_Recording_2024-12-26_at_12.15.08
don't exist
Screen Recording 2025-01-06 at 13.02.10.mov	Screen Recording 2025-01-06 at 12.59.13.mov

How to set up and validate locally

1. In rails console, enable the FF

   Feature.enable(:dast_ui_additional_variables)

2. Navigate to any project's DAST security configuration page (Secure → On-demand scans).

3. Scroll to the DAST configuration section and click Select scanner profile.

4. Add a new scanner profile, then scroll to the Additional variables (optional) section.

5. Click Add variable.

6. Validate that the UI behaves as expected.

Numbered steps to set up and validate the change are strongly suggested.

Related to #509054