[EuiDataGrid] Change header cell popover menu trigger element to be the `verticalBoxes` icon button

[tkajtoch]

Summary

This request came up from the discussion in https://github.com/elastic/eui/issues/7469.

There are cases where customers want to add interactive components like EuiTooltips to EuiDataGrid header cells to provide more context and display help text. This is currently impossible to achieve while keeping the header cells accessible. We've decided it would be worth changing the element we listen for click events on from the whole header cell to just the vertical boxes icon (with some padding added to reduce cursor/touch precision required).

Acceptance criteria

• EuiDataGrid's header cell popover menus should open only when clicking on the verticalBoxes icon button and not the whole header cell
• The updated header cell rendering implementation should be accessible
• The verticalBoxes icon button should have a data-test-subj attribute for easy referencing in automated tests

[JasonStoltz]

Some things to consider here:

• Is the click target for the menu currently large enough for accessibility?
• If we implement drag and drop for header cells, how does that interaction fit together with additional click targets in the header cell?

[ninoslavmiskovic]

We want to e.g. add ECS descriptions as tooltops for fields in the table in Discover.

E.g. for service.name

So this could be useful :)