Add workspaces agent availability to admin settings

Issue: New UI in the admin section to support mapping ... (#513370 - closed)

What does this MR do and why?

This is the first MR of the issue and includes scaffolding for the GitLab Agents Availability admin setting.

• New admin setting under Admin > Settings > General: GitLab Agents Availability.
• Skeleton of GitLab Agents Availability table
• This feature is hidden behind a WIP feature flag: workspaces_agents_availability_admin.

References

• #513370 (closed)

Screenshots or screen recordings

workspaces_agent_availability_demo

How to set up and validate locally

1. Go to /rails/features/ in the GDK and enable the workspaces_agents_availability_admin feature flag
2. Go to /admin/application_settings/general. Observe new setting GitLab Agents Availability. Expanding the setting reveals a table.

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.