feat: Custom trigger icon for button dropdown and button group menu d…

[michaeldowseza]

Description

Adds iconName, iconAlt, iconUrl, and iconSvg support to the ButtonDropdown trigger (icon and inline-icon variants) and to the ButtonGroup menu-dropdown item, falling back to the default ellipsis icon when no icon is provided.

Related links, issue #, if available: n/a

How has this been tested?

• Updated unit tests
• Updated permutations for items

• Deployed dev pages - permutations

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 97.51%. Comparing base (f968598) to head (db347a2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4628   +/-   ##
=======================================
  Coverage   97.51%   97.51%           
=======================================
  Files         948      948           
  Lines       30346    30353    +7     
  Branches    11067    11072    +5     
=======================================
+ Hits        29593    29600    +7     
- Misses        706      746   +40     
+ Partials       47        7   -40     

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