[Discover] Make Copy action visible on cell hover (elastic#204744)

- Closes elastic#203550

## Summary

This PR increases the default visible actions number from 2 to 3 so Copy
action is also included now on cell hover.

Before:
<img width="455" alt="Screenshot 2024-12-18 at 13 29 11"
src="https://github.com/user-attachments/assets/420809f4-1ed3-468a-bf23-326e91c5283d"
/>

After:
<img width="451" alt="Screenshot 2024-12-18 at 13 28 57"
src="https://github.com/user-attachments/assets/59c55abb-5228-44c4-9d00-801975b50563"
/>

src/plugins/discover/public/components/discover_grid/discover_grid.tsx

@@ -63,6 +63,7 @@ export const DiscoverGrid: React.FC<DiscoverGridProps> = ({
       renderCustomToolbar={renderCustomToolbar}
       getRowIndicator={getRowIndicator}
       rowAdditionalLeadingControls={rowAdditionalLeadingControls}
+      visibleCellActions={3} // this allows to show up to 3 actions on cell hover if available (filter in, filter out, and copy)
       {...props}
     />
   );