feat: Support custom elements in Split Panel header

[jperals]

Description

Ticket: AWSUI-18169

API proposal doc: zfC3AWNu7het

A11y review doc: M2FmAibBoga4

How has this been tested?

• Added unit test coverage
• Manually in dev page
• Ran through my pipeline to verify visual regressions
• Will provide visual regression 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]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.06%. Comparing base (bfdf4b8) to head (4d786e4).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3689   +/-   ##
=======================================
  Coverage   97.06%   97.06%           
=======================================
  Files         835      835           
  Lines       24156    24188   +32     
  Branches     8396     8486   +90     
=======================================
+ Hits        23446    23478   +32     
  Misses        662      662           
  Partials       48       48           

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