feat: Add filtering to button dropdown

[avinashbot]

Description

Button dropdown action filtering implemented a similar way to select/multiselect filtering. The big underlying change is that filtering requires the use of aria-activedescendant rather than simply moving focus from item to item. So most of the internal changes are about disabling focusing logic when filtering is active, so that the real focus can stay on the input (and "accessibility" focus can move around using aria-activedescendant).

Rel: AWSUI-61960 (Chorus: qodG8KXdJXXk)

How has this been tested?

Updated unit tests, a bit of manual accessibility testing, but more validation with other screen readers is needed.

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]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.60%. Comparing base (45502c0) to head (85a8449).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4637      +/-   ##
==========================================
+ Coverage   97.57%   97.60%   +0.02%     
==========================================
  Files         948      950       +2     
  Lines       30507    30632     +125     
  Branches    11164    11224      +60     
==========================================
+ Hits        29767    29898     +131     
- Misses        693      727      +34     
+ Partials       47        7      -40     

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