Introduce admin_runners_bulk_delete feature flag

What does this MR do and why?

This MR introduces the admin_runners_bulk_delete feature flag along with the first steps for the frontend development of this feature.

This MR:

• Adds a new checkbox column when the feature flag is on.
• Defines a new client-side field to store the "checked" state
• Responds to changed in the checkbox, to update the state.

☝ The selected checkboxes do not have an effect in our UI. However, this first step will be used in upcoming MRs.

This MR continues the work this draft MR: !81386 (closed).

Screenshots or screen recordings

How to set up and validate locally

1. Enable admin_runners_bulk_delete
   • In your command line type rails c to open the rails console
   • Type Feature.enable(:admin_runners_bulk_delete) to enable this feature
2. Visit the Admin Runners UI: http://gdk.test:3000/admin/runners

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #339525 (closed)