feat: add renderContent and visualContext props to top navigation

[amanabiy]

Description

Don't worry about the naming of the props, and that it is not a slot. Try to review the other things if possible.

Adds two new props to TopNavigation:

• renderContent: a render function that replaces all structured content (identity, search, utilities). Gives consumers full control over the navigation bar content.
• visualContext: controls whether the dark branded color scheme is applied ("top-navigation", default) or not ("none").

Additionally, identity becomes optional

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

How has this been tested?

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

❌ Patch coverage is 82.35294% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.51%. Comparing base (c158a47) to head (87d71bd).

Files with missing lines	Patch %	Lines
src/top-navigation/internal.tsx	80.64%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4594   +/-   ##
=======================================
  Coverage   97.50%   97.51%           
=======================================
  Files         947      947           
  Lines       30244    30260   +16     
  Branches    11016    11028   +12     
=======================================
+ Hits        29489    29507   +18     
+ Misses        748      746    -2     
  Partials        7        7           

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