DataTable: show sortable-column indicator on keyboard focus

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Expected Behavior

When a sortable DataTable column header receives keyboard focus, its sort affordance should be visually apparent. The indicator should communicate that the column is sortable and reflect the current or next sort direction.

Actual Behavior

The sort indicator is shown only on mouse hover. Keyboard users can tab to and activate sortable headers with Enter, but receive no visual indication that the focused header is sortable or how sorting will change.

URL, Screenshot, or CodeSandbox

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Steps to Reproduce

1. See DataTable story with sortable columns.
2. Navigate to a column header using Tab.
3. Observe that no sort indicator is visible while the header is focused.
4. Press Enter and observe that sorting is applied.
5. Hover the same header with a mouse and observe that the sort indicator becomes visible.

Your Environment

• Grommet 2.56.1