feat: Add children-based API for button group

[gethinwebster]

Description

Allow button group items to be passed as children ("composition" API). While this doesn't give functionality parity with the items API, it allows more flexibility, and still includes implementation of keyboard navigation.

Related links, issue #, if available: AWSUI-60859

How has this been tested?

New unit and integration tests

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 96.94%. Comparing base (8cda3da) to head (675c8a1). :warning: Report is 66 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3553      +/-   ##
==========================================
- Coverage   96.96%   96.94%   -0.02%     
==========================================
  Files         820      820              
  Lines       23786    23789       +3     
  Branches     8337     7921     -416     
==========================================
- Hits        23064    23063       -1     
- Misses        716      720       +4     
  Partials        6        6              

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[gethinwebster]

Closing for #3785