feat: Add headerVariant to AppLayout

[fralongo]

Description

• depends on https://github.com/cloudscape-design/components/pull/2146
• will not be merged to mainline. Several PRs will be merged at the same time in the future

How has this been tested?

Additional unit tests. Will add 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. Yes. Content review pending.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md. Yes
• Changes do not include unsupported browser features, see CONTRIBUTING.md. Yes
• Changes were manually tested for accessibility, see accessibility guidelines. N/A

Security

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

Testing

• Changes are covered with new/existing unit tests? Yes
• Changes are covered with new/existing integration tests? No

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