[terra-table] Make Buttons in Selectable Header Cells Focusable

[cm9361]

Summary

What was changed:
The Terra Table was updated such that the selectable column header cell buttons are focusable instead of the column header cells.

Why it was changed:
The change was made to be compliant with accessibility standards.

Testing

This change was tested using:

• WDIO
• Jest
• Visual testing (please attach a screenshot or recording)
• Other (please describe below)
• No tests are needed

Reviews

In addition to engineering reviews, this PR needs:

• UX review
• Accessibility review
• Functional review

Additional Details

N/A

This PR resolves:

UXPLATFORM-9802

---

Thank you for contributing to Terra.
@cerner/terra

[adoroshk]

It works as expected, but I noticed that the focus outline got 1px thicker than it was before the change. I assume it is expected as it is visible on the test screenshots, so I am approving the PR, but decided to mention just to be sure it is not something we missed by accident.

[cm9361]

It works as expected, but I noticed that the focus outline got 1px thicker than it was before the change. I assume it is expected as it is visible on the test screenshots, so I am approving the PR, but decided to mention just to be sure it is not something we missed by accident.

Yes, this is the style called out by Eric for all of our focusable elements when in focus. The table cell layout was just negatively impacting the style.