[webview_flutter] Adds NavigationDelegate.onCreateWindow for target=_blank / window.open

[mateusz-ramp]

Adds an optional NavigationDelegate.onCreateWindow callback so hosts can
handle target=_blank / window.open (e.g. open in an external browser)
instead of only loading in the same WebView.

Behavior

• If onCreateWindow is set: Android (WebChromeClient.onCreateWindow →
  pigeon) and iOS/macOS (WKUIDelegate.createWebViewWith) invoke the callback
  with the requested URL.
• If unset: preserves existing same-WebView load behavior.

Packages (federated review PR)

• webview_flutter_platform_interface 2.16.0
• webview_flutter_android 4.14.0
• webview_flutter_wkwebview 3.27.0
• webview_flutter 4.15.0

Path dependency_overrides are present for combined review only
(make-deps-path-based). This PR is not intended to land as-is; after
approval we will land/publish in the usual federated order (platform
interface → implementations → app-facing).

Related issues

Related to:

• [WebView] Add support for target property of anchor tag flutter#95060
• [webview_flutter] Support spawning new webviews from JS flutter#36052
• [webview_flutter] [iOS] Clicking on a Paypal Payment button does not open paypal flutter#93425

Background (prior same-window _blank handling):

• webview_flutter target _blank is not working flutter#28875
• [webview_flutter] iOS can not open target _blank links and links inside iframes flutter#42448
• [webview_flutter] Fix iOS WebView not opening href with target="_blank" flutter-team-archive/plugins#2500

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• I signed the [CLA].
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• I [linked to at least one issue that this PR fixes] in the description above.
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