[EuiContextMenuPanelDescriptor] Allow specifying initialFocusedItemIndex for panel

[flash1293]

If menu panels are constructed manually by the consumer, the first item to focus can be set by using initialFocusedItemIndex. However if using the EuiContextMenuPanelDescriptor approach (passing panels into an EuiContextMenu element), it's automatically set to 0 if keyboard navigation is used which might not always be the desired behavior (e.g. elastic/kibana#81058)

I can imagine extending EuiContextMenuPanelDescriptor by initialFocusedItemIndex and using that in favor of 0 if set.

This is where the new prop would be hooked in
https://github.com/elastic/eui/blob/master/src/components/context_menu/context_menu.tsx#L388