Description
I'd like to request we add two optional props to the <HeaderLinks /> component:

1. A ref hook of type <HTMLElement>
2. A tabindex with default value of -1

I'm reviewing an issue in Kibana where focus is not managed properly on the dashboard views when users click the Edit/Save button to switch from View to Edit mode and back again. The larger issue is Kibana-specific, but these props will enable consumers to manage focus with custom event handlers.

Success criteria

• The ref may only need to be added to our EuiHeaderLinksProps type
• tabindex="-1" could be a boolean like isFocusable that sets "-1" or only if the prop is passed
• No focus halo when the element takes focus. I'm happy to discuss this one further, because it's a gray area.