Add modal to add tracked refs

What does this MR do and why?

This MR adds a modal that allows users to select and track new code references (branches and tags) in a security configuration interface.

Note: To keep the changes small and manageable, this is only the first in a series of MRs, and it focuses on:

1. Creating and setting up the modal
2. Rendering the initial list of selectable refs
3. Adding the search-input and implementing a rudimentary search

Future MRs will tackle:

• GraphQL fetching (initial data and searching)
• Loading and error states
• Keeping track of selected refs
• Showing how many refs can be selected and warning the user when the limit is reached
• Submitting the selected refs

Before	After

How to set up and validate locally

1. Enable the feature flag: http://gdk.test:3000/rails/features/vulnerabilities_across_contexts
2. Navigate to a project's security configuration
3. Open the "Vulnerability Management" tab
4. Click on "Track new refs"
5. Verify that the modal opens, a list of refs is rendered, refs can be selected, and the input does a search

MR acceptance checklist

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

Related to #577515