Add support for WebAuthenticationSession's additionalHeaderFields (iOS 17.4+, macOS 14.4+)

[schulzp]

Connection with issue

Resolve issue #2835

Testing and Review Notes

• Run an app build on iOS 17.4+ or macOS 14.4+.
• Create a WebAuthenticationSession with additionalHeaderFields, for example:
  {'X-Test-Header': 'test-value'}
• Use an initial URL that points to a request inspection service such as:
  https://echo.free.beeceptor.com/
• Start the session and verify that the custom header is present in the outgoing initial request.
• Repeat on an older iOS/macOS version and verify that the session still works without crashing, with the header being ignored on unsupported OS versions.

To Do

• Confirm the preferred long-term test location for this feature.
• Decide whether the current Dart-side package tests are sufficient, or whether this should be covered by an example/integration test against a real endpoint.

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