Add a loading spinner to the contact not loaded placeholder component

[stephenhand]

Description

• Add a loading spinner to the ContentNotLoaded component in plugin-hrm-form
• It should be vertically and horizontally centered within this component
• It should not interfere with the positioning, visibility or functionality of the existing text in that component
• Ensure the spinner is fully accessible

Additional context

We have circular spinners in use in other parts of the UI already, reuse this