[EuiDataGrid][KEYBOARD]: Update data grid control cells' focus halo to same blue as other cells

[1Copenut]

While testing the new Discover data grid, I noticed the control columns were using a black focus halo for the checkboxes and flyout control. I wanted to ask if it made sense to make this focus halo blue for consistency with other cells.

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[elizabetdev]

@1Copenut yes, it would make sense. It creates more consistency.

[Al-0]

Hi, I will submit a PR to fix this!