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

[tkajtoch]

Summary

This request came up from the discussion in #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