Add a property on WebView to set additional headers

### Description

On the platform specific WebView you can launch the browser view with some additional headers set (for example for authentication). Maui WebView doesn't have this capability at the moment and you must override handlers mapping for each platform

### Public API Changes

```xml

```

### Intended Use-Case

Simplify webview automatic authentication

Start by tracing the platform-specific WebView handler mappings and the existing WebView public API. Check how a bound Headers property could reach each native browser view; done means the property supports the intended authentication use case across supported platforms and has corresponding coverage.

Written by the indexing model from the issue text.