Add Support for Custom Rack Fields in Rack List View

Allow users to define and display custom fields for racks in the list view under Companies > [Company Name] > Racks.

Currently, the Rack list view only shows default fields such as Name, Description, and Utilization.

We would like the ability to:

1. Add custom fields to rack entries (e.g., location, notes, tags, etc.)

2. Choose which of those custom fields are visible as columns in the list view

3. Improve filtering, searching, and visibility based on custom data