Add ProxyController support for Windows (WebView2)

## Description ProxyController is currently implemented for Android, iOS 17+, macOS 14+, and Linux, but **Windows is missing**. On Windows, `ProxyController.setProxyOverride()` throws `UnimplementedError`. ## Proposed Solution WebView2 supports proxy via `--proxy-server` and `--proxy-bypass-list` browser arguments passed through `additionalBrowserArguments` in `ICoreWebView2EnvironmentOptions`. The implementation: 1. **Native C++ `ProxyManager`** — listens on method channel `com.pichillilorenzo/flutter_inappwebview_proxycontroller`, stores proxy settings, and injects `--proxy-server`/`--proxy-bypass-list` into `CoreWebView2EnvironmentOptions` when `WebViewEnvironment::create()` is called. 2. **Dart `WindowsProxyController`** — extends `PlatformProxyController` with `ChannelController` mixin, registered in `WindowsInAppWebViewPlatform`. 3. **Default environment routing** — `InAppWebView.createInAppWebViewEnv()` fallback changed from raw `CreateCoreWebView2EnvironmentWithOptions(nullptr, nullptr, nullptr, ...)` to `WebViewEnvironmentManager::createOrGetDefaultWebViewEnvironment()`, so proxy args are picked up even when no explicit `WebViewEnvironment` is passed. 4. **Platform Interface** — `WindowsPlatform` added to `@SupportedPlatforms` annotations for `ProxyController`, `ProxySettings`, `setProxyOverride`, `clearProxyOverride`, `bypassRules`, and `proxyRules`. ## Limitations - Proxy is set via browser arguments at environment creation time. It cannot be changed at runtime for an already-created WebView2 environment (WebView2 limitation). - `setProxyOverride()` must be called **before** the WebView is created. ## Platform Support After This Change | Platform | Status | API | |----------|--------|-----| | Android | ✅ | `androidx.webkit.ProxyController` | | iOS 17+ | ✅ | `WKWebsiteDataStore.proxyConfigurations` | | macOS 14+ | ✅ | `WKWebsiteDataStore.proxyConfigurations` | | Linux | ✅ | `WebKitNetworkProxySettings` | | **Windows** | **✅ (new)** | `WebView2 --proxy-server` | I have a working implementation ready and will open a PR.