Expose WebkitGTK Proxy Feature on Linux

[tillmann-crabnebula]

Currently wry does not expose or support upstream proxies for the webview on Linux.
WebkitGTK supports this out of the box since version 2.16 but it is not exposed in a uniform interface in wry.

I would like to have an exposed wry proxy configuration, where it is possible to set the webview proxy independent
of the underlying WebkitGTK method. Possibly this interface could be used for Webview2 by adding custom parameters
to the window creation by default.
I would like to have a patch for older wry versions, so we could use this feature in Tauri 1.x instead of just in the 2.x releases.

The most recent supported interface is exposed in the WebsiteDataManager.set_network_proxy_settings
and the deprecated version is exposed via WebContext.set_network_proxy_settings.

There are no alternatives present and using this for auditing or compliance would require a custom built version of Tauri and Wry.
The functionality is needed in corporate environments where there are upstream proxies to be honored and when auditing Tauri applications it is helpful to use common security tooling in the process.

The proxy feature is exposed on windows via the custom webview arguments (--proxy-server 127.0.0.1:8080) and on MacOs in a new beta version.

Discussed this shortly with @wusyong before opening the issue here.

[pewsheen]

https://developer.apple.com/videos/play/wwdc2023/10002/

This might help us to understand proxyConfigurations on macOS

[pewsheen]

WebKit documentation for setting up the proxy configuration:
https://developer.apple.com/documentation/webkit/wkwebsitedatastore/4245799-proxyconfigurations?language=objc

[dklassic]

Surveying WebView2 proxy feature:

• How to use Microsoft Edge command-line options to configure proxy settings
• Support for per-request proxy configuration MicrosoftEdge/WebView2Feedback#132
• Questions: Page Crash Event ?, Proxy per application ? MicrosoftEdge/WebView2Feedback#193